Introduce a new header `message/rpc_protocol_impl.hh`, move here
the following things from `message/messaging_service.cc`:
* RPC protocol wrappers implementation
* Serialization thunks
* `register_handler` and `send_message*` functions
This code will be used later for IDL-generated RPC verbs
implementation.
Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com>
