gRPC query improvements

[chipshort]

It is incompatible with QuerierWrapper's query because query requires a json-decodable response (but the gRPC query is protobuf). query_raw works, but requires already-serialized input and is hard to use.

• Document this on the gRPC query (and maybe add sample code on how to query)
• Refactor out the part of the query function without the from_json call (that's what fails for gRPC).
• Maybe we want to provide a direct query_grpc(path, data) -> Binary function using the refactored out part?

[webmaster128]

Let's do that. It's just a shame that the nice name "query" is taken already and cannot be used for the more general case.