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

As per conversation here: https://github.com/tendermint/tendermint/pull/3218#discussion_r251364041

This is the result of running the following code on the repo:

```bash
find . -name '*.go' | grep -v 'vendor/' | xargs -n 1 goimports -w
```

* Fixed accepting integer IDs in requests for Tendermint RPC server (#2366)

* added a wrapper interface `jsonrpcid` that represents both string and int IDs in JSON-RPC requests/responses + custom JSON unmarshallers

* changed client-side code in RPC that uses it

* added extra tests for integer IDs

* updated CHANGELOG_PENDING, as suggested by PR instructions

* addressed PR comments

* added table driven tests for request type marshalling/unmarshalling
* expanded handler test to check IDs
* changed pending changelog note

* changed json rpc request/response unmarshalling to use empty interfaces and type switches on ID

* some cleanup

also, return ErrNotStarted when trying to stop a not-running service

* libs: Remove usage of custom Fmt, in favor of fmt.Sprintf

Closes #2193

* Fix bug that was masked by custom Fmt!

This reverts commit d02c5d1e30.

* Implement support for https in rpc client

* Update changelog

* update comment

* support wss protocol

* fixed changelog

* if -> switch

* fix lint

* protocol constants introduced

This reverts commit 96a3502

* replace math/rand with tmlibs equivalent.

* update tmlibs dependency

* use increment and decrement operators.

* remove unnecessary else branches.

* fix package comment with leading space.

* fix receiver names.

* fix error strings.

* remove omittable code.

* remove redundant return statement.

* Revert changes (code is generated.)

* use cfg as receiver name for all config-related types.

* use lsi as the receiver name for the LastSignedInfo type.

* remove accidental binary

* docs: s/Dummy&dummy/KVStore&kvstore/g

* glide update to abci

* update abci import paths

* dummy begone, hello kvstore

* RequestInitChain needs genesisBytes

* glide update

1. remove pointer
2. add Quit() method to Service interface

* linter: address gosimple lints

* linter: make deterministic & a rebase fix

* lint/rpc: fix a gosimple lint

* run linter in CI

* fix rebase mistake

* fix makefile

* ugh

* revert Makefile

* add metalinter to CI

* try this

* linter: last little fix

* need glide

* better

* okayy circle, have it your way

* lints: gosimple

* pr comments

See https://github.com/tendermint/tmlibs/issues/45

comment out failing consensus tests for now

rewrite rpc httpclient to use new pubsub package

import pubsub as tmpubsub, query as tmquery

make event IDs constants
EventKey -> EventTypeKey

rename EventsPubsub to PubSub

mempool does not use pubsub

rename eventsSub to pubsub

new subscribe API

fix channel size issues and consensus tests bugs

refactor rpc client

add missing discardFromChan method

add mutex

rename pubsub to eventBus

remove IsRunning from WSRPCConnection interface (not needed)

add a comment in broadcastNewRoundStepsAndVotes

rename registerEventCallbacks to broadcastNewRoundStepsAndVotes

See https://dave.cheney.net/2014/03/19/channel-axioms

stop eventBuses after reactor tests

remove unnecessary Unsubscribe

return subscribe helper function

move discardFromChan to where it is used

subscribe now returns an err

this gives us ability to refuse to subscribe if pubsub is at its max
capacity.

use context for control overflow

cache queries

handle err when subscribing in replay_test

rename testClientID to testSubscriber

extract var

set channel buffer capacity to 1 in replay_file

fix byzantine_test

unsubscribe from single event, not all events

refactor httpclient to return events to appropriate channels

return failing testReplayCrashBeforeWriteVote test

fix TestValidatorSetChanges

refactor code a bit

fix testReplayCrashBeforeWriteVote

add comment

fix TestValidatorSetChanges

fixes from Bucky's review

update comment [ci skip]

test TxEventBuffer

update changelog

fix TestValidatorSetChanges (2nd attempt)

only do wg.Done when no errors

benchmark event bus

create pubsub server inside NewEventBus

only expose config params (later if needed)

set buffer capacity to 0 so we are not testing cache

new tx event format: key = "Tx" plus a tag {"tx.hash": XYZ}

This should allow to subscribe to all transactions! or a specific one
using a query: "tm.events.type = Tx and tx.hash = '013ABF99434...'"

use TimeoutCommit instead of afterPublishEventNewBlockTimeout

TimeoutCommit is the time a node waits after committing a block, before
it goes into the next height. So it will finish everything from the last
block, but then wait a bit. The idea is this gives it time to hear more
votes from other validators, to strengthen the commit it includes in the
next block. But it also gives it time to hear about new transactions.

waitForBlockWithUpdatedVals

rewrite WAL crash tests

Task:
test that we can recover from any WAL crash.

Solution:
the old tests were relying on event hub being run in the same thread (we
were injecting the private validator's last signature).

when considering a rewrite, we considered two possible solutions: write
a "fuzzy" testing system where WAL is crashing upon receiving a new
message, or inject failures and trigger them in tests using something
like https://github.com/coreos/gofail.

remove sleep

no cs.Lock around wal.Save

test different cases (empty block, non-empty block, ...)

comments

add comments

test 4 cases: empty block, non-empty block, non-empty block with smaller part size, many blocks

fixes as per Bucky's last review

reset subscriptions on UnsubscribeAll

use a simple counter to track message for which we panicked

also, set a smaller part size for all test cases

* Updated code with feedback from @melekes, @ebuchman and @silasdavis.
* Added Makefile clause `release` to only run the test on seeing tag
`release` during releases i.e
```shell
make release
```
which will run the comprehensive and long integration-ish tests.

Fixes https://github.com/tendermint/tendermint/issues/751.

Adds jitter to our exponential backoff to mitigate a self DDOS
vector. The jitter is a randomly picked percentage of a second
whose purpose is to ensure that each exponential backoff retry
occurs within (1<<attempts) == 2**attempts, but with the delay
each client will have a random buffer time before it tries to
reconnect instead of all at once reconnections that might even
bring back the previous conditions that might have caused the
dial to the WSServer to have failed e.g
* Network outage
* File descriptor exhaustion
* False positives from firewalls
etc

This cleans up some of the code in the state package

callback gives more power to the publisher. plus it is optional
comparing to a channel, which will block the whole client if you won't
read from it.

* rpc: subscribe on reconnection

* rpc: fix unit tests

server:
- always has read & write timeouts
- ping handler never blocks the reader (see A)
- sends regular pings to check up on a client

A:
at some point server write buffer can become full, so in order not to
block reads from a client (see
https://github.com/gorilla/websocket/issues/97), server may skip some
pongs. As a result, client may disconnect. But you either have to do
that or block the reader. There is no third way.

client:
- optional read & write timeouts
- optional ping/pong to measure latency