Consider externalizing queues from RpcPeerProtocol to separate classes

During review, it was noted that RpcPeerProtocol has too many responsibilities and should be split into smaller classes - in particular, queues should be external, so they can be injected into RpcPeerProtocol and RpcPeerHandler and be a communication point between them, rather than having everything in single place. This will add considerable complexity to the system, but might result in cleaner design.