What types of RPC transactions are there?
* method: An RPC call with one or more return values * method_return: An RPC call with the return values from a previous RPC method call. This is best thought of as a “callback”. Callbacks have their own argument list (e.g. method signature) completely independent of the signature of its matching method. * signal: An RPC call which does not return any values (e.g. a one way event) * error_return: An RPC call whose signature is (method_name, err_code, err_msg).
