Batch mode

[mehaase]

Section 6 of the JSON-RPC spec defines "batch mode", where multiple request are sent in a single message. It's kind of a silly part of the spec, but in order to be compliant we do need to support it. It complicates the API design since the spec allows for requests in a batch to be processed concurrently, which means we probably can't abstract over the difference between a batch and a single request. We'll have to expose this detail to downstream implementations.