Major Description
It would be nice to allow for multiple RPC services on the same port. This would allow reuse of common RPC service interfaces and implementations between, for example, the tablet server and the master.
This is fairly easy to implement. We need to add an RPC service name field to the RPC request header protobuf, and implement the TODO in Messenger::RegisterService() to support multiple RpcService implementations in a Messenger. Then, Messenger::QueueInboundCall() can key on the service name in the InboundCall request header to route the request to the right service.