Fixes #4739, kind of. See #4740 for the proper fix.

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For contributor use:

- [x] Wrote tests
- [x] Updated CHANGELOG_PENDING.md
- [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [x] Updated relevant documentation (`docs/`) and code comments
- [x] Re-reviewed `Files changed` in the Github PR explorer
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Closes: #4530

This PR contains logic for both submitting an evidence by the light client (lite2 package) and receiving it on the Tendermint side (/broadcast_evidence RPC and/or EvidenceReactor#Receive). Upon receiving the ConflictingHeadersEvidence (introduced by this PR), the Tendermint validates it, then breaks it down into smaller pieces (DuplicateVoteEvidence, LunaticValidatorEvidence, PhantomValidatorEvidence, PotentialAmnesiaEvidence). Afterwards, each piece of evidence is verified against the state of the full node and added to the pool, from which it's reaped upon block creation.

* rpc/client: do not pass height param if height ptr is nil

* rpc/core: validate incoming evidence!

* only accept ConflictingHeadersEvidence if one

of the headers is committed from this full node's perspective

This simplifies the code. Plus, if there are multiple forks, we'll
likely to receive multiple ConflictingHeadersEvidence anyway.

* swap CommitSig with Vote in LunaticValidatorEvidence

Vote is needed to validate signature

* no need to embed client

http is a provider and should not be used as a client

Closes: #4643

* rpc/client: initial split into directories

* lite2: split out test package

* rpc/client: simplify client constructurs

* updated docs

* updated changelog

Closes #4385

* extract TrustOptions into its own file

* print trusted hash before asking whenever to rollback or not

so the user could reset the light client with the trusted header

* do not return an error if rollback is aborted

reason: we trust the old header presumably, so can continue from it.

* add note about time of initial header

* improve logging and add comments

* cross-check newHeader after LC verified it

* check if header is not nil

so we don't crash on the next line

* remove witness if it sends us incorrect header

* require at least one witness

* fix build and tests

* rename tests and assert for specific error

* wrote a test

* fix linter errors

* only check 1/3 if headers diverge

before:
&http{AFBSD743A...}

after:
http{https://127.0.0.1:26657}

Co-authored-by: Marko <marbar3778@yahoo.com>

Closes issue #4338

Uses a wrapper function around both the signedHeader and validatorSet calls to the primary provider which attempts to retrieve the information 5 times before deeming the provider unavailable and replacing the primary provider with the first alternative before trying recursively again (until all alternatives are depleted)

Employs a mutex lock for any operations involving the providers of the light client to ensure no operations occurs whilst the new primary is chosen.

Commits:

* created swapProvider function

* eliminates old primary provider after replacement. Uses a mutex when changing providers

* renamed to replaceProvider

* created wrapped functions for signed header and val set

* created test for primary provider replacement

* implemented suggested revisions

* created Witnesses() and Primary()

* modified backoffAndJitterTime

* modified backoffAndJitterTime

* changed backoff base and jitter to functional arguments

* implemented suggested changes

* removed backoff function

* changed exp function to match go version

* halved the backoff time

* removed seeding and added comments

* fixed incorrect test

* extract backoff timeout calc into a function

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>

* lite2: advance to latest header

without any exponential steps

rename autoUpdate to autoUpdateRoutine

* lite2: wait in Cleanup until goroutines finished running

* lite2: move AutoClient into Client

Most of the users will want auto update feature, so it makes sense to
move it into the Client itself, rather than having a separate
abstraction (it makes the code cleaner, but introduces an extra thing
the user will need to learn).

Also, add `FirstTrustedHeight` func to Client to get first trusted height.

* fix db store tests

* separate examples for auto and manual clients

* AutoUpdate tries to update to latest state

NOT 1 header at a time

* fix errors

* lite2: make Logger an option

remove SetLogger func

* fix lite cmd

* lite2: make concurrency assumptions explicit

* fixes after my own review

* no need for nextHeightFn

sequence func will download intermediate headers

* correct comment

* rename adjusted to adjacent

Refs https://github.com/tendermint/tendermint/pull/3989#discussion_r352140829

* rename ErrTooMuchChange to ErrNotEnoughVotingPowerSigned

Refs https://github.com/tendermint/tendermint/pull/3989#discussion_r352142785

* verify commit is properly signed

* remove no longer trusted headers

* restore trustedHeader and trustedNextVals

* check trustedHeader using options

Refs https://github.com/tendermint/tendermint/pull/4209#issuecomment-562462165

* use correct var when checking if headers are adjacent

in bisection func
+ replace TODO with a comment

https://github.com/tendermint/tendermint/pull/3989#discussion_r352125455

* return header in VerifyHeaderAtHeight

because that way we avoid DB call

+ add godoc comments
+ check if there are no headers yet in AutoClient

https://github.com/tendermint/tendermint/pull/3989#pullrequestreview-315454506

* TestVerifyAdjacentHeaders: add 2 more test-cases

+ add TestVerifyReturnsErrorIfTrustLevelIsInvalid

* lite: avoid overflow when parsing key in db store!

* lite: rename AutoClient#Err to Errs

* lite: add a test for AutoClient

* lite: fix keyPattern and call itr.Next in db store

* lite: add two tests for db store

* lite: add TestClientRemovesNoLongerTrustedHeaders

* lite: test Client#Cleanup

* lite: test restoring trustedHeader

https://github.com/tendermint/tendermint/pull/4209#issuecomment-562462165

* lite: comment out unused code in test_helpers

* fix TestVerifyReturnsErrorIfTrustLevelIsInvalid after merge

* change defaultRemoveNoLongerTrustedHeadersPeriod

and add docs

* write more doc

* lite: uncomment testable examples

* use stdlog.Fatal to stop AutoClient tests

* make lll linter happy

* separate errors for 2 cases

- the validator set of a skipped header cannot be trusted, i.e. <1/3rd
  of h1 validator set has signed (new error, something like
  ErrNewValSetCantBeTrusted)
- the validator set is trusted but < 2/3rds has signed
  (ErrNewHeaderCantBeTrusted)

https://github.com/tendermint/tendermint/pull/4209#discussion_r360331253

* remove all headers (even the last one) that are outside

of the trusting period. By doing this, we avoid checking the
trustedHeader's hash in checkTrustedHeaderUsingOptions (case #1).

https://github.com/tendermint/tendermint/pull/4209#discussion_r360332460

* explain restoreTrustedHeaderAndNextVals better

https://github.com/tendermint/tendermint/pull/4209#discussion_r360602328

* add ConfirmationFunction option

for optionally prompting for user input Y/n before removing headers

Refs https://github.com/tendermint/tendermint/pull/4209#discussion_r360602945

* make cleaning optional

https://github.com/tendermint/tendermint/pull/4209#discussion_r364838189

* return error when user refused to remove headers

* check for double votes in VerifyCommitTrusting

* leave only ErrNewValSetCantBeTrusted error

to differenciate between h2Vals.VerifyCommit and
h1NextVals.VerifyCommitTrusting

* fix example tests

* remove unnecessary if condition

https://github.com/tendermint/tendermint/pull/4209#discussion_r365171847

It will be handled by the above switch.

* verifyCommitBasic does not depend on vals

Co-authored-by: Marko <marbar3778@yahoo.com>

The New* client functions return an error instead
of panicking when the remote address is invalid.

Fixes #3953

* Fix many golint errors

* Fix golint errors in the 'lite' package

* Don't export Pool.store

* Fix typo

* Revert unwanted changes

* Fix errors in counter package

* Fix linter errors in kvstore package

* Fix linter error in example package

* Fix error in tests package

* Fix linter errors in v2 package

* Fix linter errors in consensus package

* Fix linter errors in evidence package

* Fix linter error in fail package

* Fix linter errors in query package

* Fix linter errors in core package

* Fix linter errors in node package

* Fix linter errors in mempool package

* Fix linter error in conn package

* Fix linter errors in pex package

* Rename PEXReactor export to Reactor

* Fix linter errors in trust package

* Fix linter errors in upnp package

* Fix linter errors in p2p package

* Fix linter errors in proxy package

* Fix linter errors in mock_test package

* Fix linter error in client_test package

* Fix linter errors in coretypes package

* Fix linter errors in coregrpc package

* Fix linter errors in rpcserver package

* Fix linter errors in rpctypes package

* Fix linter errors in rpctest package

* Fix linter error in json2wal script

* Fix linter error in wal2json script

* Fix linter errors in kv package

* Fix linter error in state package

* Fix linter error in grpc_client

* Fix linter errors in types package

* Fix linter error in version package

* Fix remaining errors

* Address review comments

* Fix broken tests

* Reconcile package coregrpc

* Fix golangci bot error

* Fix new golint errors

* Fix broken reference

* Enable golint linter

* minor changes to bring golint into line

* fix failing test

* fix pex reactor naming

* address PR comments

Refs #1771

ADR: https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-044-lite-client-with-weak-subjectivity.md

## Commits:

* add Verifier and VerifyCommitTrusting

* add two more checks

make trustLevel an option

* float32 for trustLevel

* check newHeader time

* started writing lite Client

* unify Verify methods

* ensure h2.Header.bfttime < h1.Header.bfttime + tp

* move trust checks into Verify function

* add more comments

* more docs

* started writing tests

* unbonding period failures

* tests are green

* export ErrNewHeaderTooFarIntoFuture

* make golangci happy

* test for non-adjusted headers

* more precision

* providers and stores

* VerifyHeader and VerifyHeaderAtHeight funcs

* fix compile errors

* remove lastVerifiedHeight, persist new trusted header

* sequential verification

* remove TrustedStore option

* started writing tests for light client

* cover basic cases for linear verification

* bisection tests PASS

* rename BisectingVerification to SkippingVerification

* refactor the code

* add TrustedHeader method

* consolidate sequential verification tests

* consolidate skipping verification tests

* rename trustedVals to trustedNextVals

* start writing docs

* ValidateTrustLevel func and ErrOldHeaderExpired error

* AutoClient and example tests

* fix errors

* update doc

* remove ErrNewHeaderTooFarIntoFuture

This check is unnecessary given existing a) ErrOldHeaderExpired b)
h2.Time > now checks.

* return an error if we're at more recent height

* add comments

* add LastSignedHeaderHeight method to Store

I think it's fine if Store tracks last height

* copy over proxy from old lite package

* make TrustedHeader return latest if height=0

* modify LastSignedHeaderHeight to return an error if no headers exist

* copy over proxy impl

* refactor proxy and start http lite client

* Tx and BlockchainInfo methods

* Block method

* commit method

* code compiles again

* lite client compiles

* extract updateLiteClientIfNeededTo func

* move final parts

* add placeholder for tests

* force usage of lite http client in proxy

* comment out query tests for now

* explicitly mention tp: trusting period

* verify nextVals in VerifyHeader

* refactor bisection

* move the NextValidatorsHash check into updateTrustedHeaderAndVals

+ update the comment

* add ConsensusParams method to RPC client

* add ConsensusParams to rpc/mock/client

* change trustLevel type to a new cmn.Fraction type

+ update SkippingVerification comment

* stress out trustLevel is only used for non-adjusted headers

* fixes after Fede's review

Co-authored-by: Federico Kunze <31522760+fedekunze@users.noreply.github.com>

* compare newHeader with a header from an alternative provider

* save pivot header

Refs https://github.com/tendermint/tendermint/pull/3989#discussion_r349122824

* check header can still be trusted in TrustedHeader

Refs https://github.com/tendermint/tendermint/pull/3989#discussion_r349101424

* lite: update Validators and Block endpoints

- Block no longer contains BlockMeta
- Validators now accept two additional params: page and perPage

* make linter happy