rpclib: Use JsonValue to re-encode responses and requests.

This approach keeps the property that we still preserve any unrecognized fields, but takes a different approach. Instead of using our own structs to round-trip the json, we use a serde_json::Value, to ensure that we cannot forget to add the unexpected_fields element to a struct.

Closes #1512 (closed).

(Also closes #1511 (closed) with a rename.)