Generic RPC handling (used for both front end and plugin communication).
The RPC protocol is based on JSON-RPC, but with some modifications. Unlike JSON-RPC 2.0, requests and notifications are allowed in both directions, rather than imposing client and server roles. Further, the batch form is not supported.
Because these changes make the protocol not fully compliant with the spec,
the "jsonrpc" member is omitted from request and response objects.