JSON-RPC batching

[zombiezen]

I skipped implementing the batching section of the JSON-RPC specification, but it would be nice to implement that for some of the more bursty RPCs. I think it would reduce wire overhead, since it would be less frames that would need to cross the wire.

[zombiezen]

Actually, came up with a more interesting use case. In the case where the frontend wants multiple derivations to be realized, we can schedule the work more efficiently as a batch, while keeping the API the same.