Fixes #828. Adds state sync, as outlined in [ADR-053](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-053-state-sync-prototype.md). See related PRs in Cosmos SDK (https://github.com/cosmos/cosmos-sdk/pull/5803) and Gaia (https://github.com/cosmos/gaia/pull/327).

This is split out of the previous PR #4645, and branched off of the ABCI interface in #4704. 

* Adds a new P2P reactor which exchanges snapshots with peers, and bootstraps an empty local node from remote snapshots when requested.

* Adds a new configuration section `[statesync]` that enables state sync and configures the light client. Also enables `statesync:info` logging by default.

* Integrates state sync into node startup. Does not support the v2 blockchain reactor, since it needs some reorganization to defer startup.

* Added BlockStore.DeleteBlock()

* Added initial block pruner prototype

* wip

* Added BlockStore.PruneBlocks()

* Added consensus setting for block pruning

* Added BlockStore base

* Error on replay if base does not have blocks

* Handle missing blocks when sending VoteSetMaj23Message

* Error message tweak

* Properly update blockstore state

* Error message fix again

* blockchain: ignore peer missing blocks

* Added FIXME

* Added test for block replay with truncated history

* Handle peer base in blockchain reactor

* Improved replay error handling

* Added tests for Store.PruneBlocks()

* Fix non-RPC handling of truncated block history

* Panic on missing block meta in needProofBlock()

* Updated changelog

* Handle truncated block history in RPC layer

* Added info about earliest block in /status RPC

* Reorder height and base in blockchain reactor messages

* Updated changelog

* Fix tests

* Appease linter

* Minor review fixes

* Non-empty BlockStores should always have base > 0

* Update code to assume base > 0 invariant

* Added blockstore tests for pruning to 0

* Make sure we don't prune below the current base

* Added BlockStore.Size()

* config: added retain_blocks recommendations

* Update v1 blockchain reactor to handle blockstore base

* Added state database pruning

* Propagate errors on missing validator sets

* Comment tweaks

* Improved error message

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

* use ABCI field ResponseCommit.retain_height instead of retain-blocks config option

* remove State.RetainHeight, return value instead

* fix minor issues

* rename pruneHeights() to pruneBlocks()

* noop to fix GitHub borkage

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

* Add test for bcBlockRequestMessage ValidateBasic method

* Add test for bcNoBlockResponseMessage ValidateBasic method

* Add test for bcStatusRequestMessage ValidateBasic method

* Add test for bcStatusResponseMessage ValidateBasic method

* Add blockchain v1 reactor ValidateBasic tests

* Add test for NewRoundStepMessage ValidateBasic method

* Add test for NewValidBlockMessage ValidateBasic method

* Test BlockParts Size

* Import cmn package

* Add test for ProposalPOLMessage ValidateBasic method

* Add test for BlockPartMessage ValidateBasic method

* Add test for HasVoteMessage ValidateBasic method

* Add test for VoteSetMaj23Message ValidateBasic method

* Add test for VoteSetBitsMessage ValidateBasic method

* Fix linter errors

* Improve readability

* Add test for BaseConfig ValidateBasic method

* Add test for RPCConfig ValidateBasic method

* Add test for P2PConfig ValidateBasic method

* Add test for MempoolConfig ValidateBasic method

* Add test for FastSyncConfig ValidateBasic method

* Add test for ConsensusConfig ValidateBasic method

* Add test for InstrumentationConfig ValidateBasic method

* Add test for BlockID ValidateBasic method

* Add test for SignedHeader ValidateBasic method

* Add test for MockGoodEvidence and MockBadEvidence ValidateBasic methods

* Remove debug logging

Co-Authored-By: Marko <marbar3778@yahoo.com>

* Update MempoolConfig field

* Test a single struct field at a time, for maintainability

Fixes #2740

* add missing options to config.toml template and docs
Refs #2232
* config#ValidateBasic
Refs #2232
* [config] timeouts as time.Duration, not ints
Why:
- native type provides better guarantees than ", in ms" comment (harder
to shoot yourself in the leg)
- flexibility: you can change units

This reverts commit d02c5d1e30.

This reverts commit 96a3502