Code review comment for lp:~kdub/mir/prepare-for-display-notifications

@alexandros, wrote a test (test_socket_session.cpp) to tease out the receiving functionality from the socket session, and ended up with a class structure very close to the suggestion. Also, we now initialize the client resources after the accept (as the acceptor needed the socket to accept the connection, then we transfer the socket to the SocketMessenger class for the lifetime of the connection.