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
