* rpc/client: initial split into directories

* lite2: split out test package

* rpc/client: simplify client constructurs

* updated docs

* updated changelog

but HTTP client is not running.

`Subscribe`, `Unsubscribe(All)` methods return an error now.

Closes #4568

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

Continues from #3280 in building support for batched requests/responses in the JSON RPC (as per issue #3213).

* Add JSON RPC batching for client and server

As per #3213, this adds support for [JSON RPC batch requests and
responses](https://www.jsonrpc.org/specification#batch).

* Add additional checks to ensure client responses are the same as results

* Fix case where a notification is sent and no response is expected

* Add test to check that JSON RPC notifications in a batch are left out in responses

* Update CHANGELOG_PENDING.md

* Update PR number now that PR has been created

* Make errors start with lowercase letter

* Refactor batch functionality to be standalone

This refactors the batching functionality to rather act in a standalone
way. In light of supporting concurrent goroutines making use of the same
client, it would make sense to have batching functionality where one
could create a batch of requests per goroutine and send that batch
without interfering with a batch from another goroutine.

* Add examples for simple and batch HTTP client usage

* Check errors from writer and remove nolinter directives

* Make error strings start with lowercase letter

* Refactor examples to make them testable

* Use safer deferred shutdown for example Tendermint test node

* Recompose rpcClient interface from pre-existing interface components

* Rename WaitGroup for brevity

* Replace empty ID string with request ID

* Remove extraneous test case

* Convert first letter of errors.Wrap() messages to lowercase

* Remove extraneous function parameter

* Make variable declaration terse

* Reorder WaitGroup.Done call to help prevent race conditions in the face of failure

* Swap mutex to value representation and remove initialization

* Restore empty JSONRPC string ID in response to prevent nil

* Make JSONRPCBufferedRequest private

* Revert PR hard link in CHANGELOG_PENDING

* Add client ID for JSONRPCClient

This adds code to automatically generate a randomized client ID for the
JSONRPCClient, and adds a check of the IDs in the responses (if one was
set in the requests).

* Extract response ID validation into separate function

* Remove extraneous comments

* Reorder fields to indicate clearly which are protected by the mutex

* Refactor for loop to remove indexing

* Restructure and combine loop

* Flatten conditional block for better readability

* Make multi-variable declaration slightly more readable

* Change for loop style

* Compress error check statements

* Make function description more generic to show that we support different protocols

* Preallocate memory for request and result objects