Code review comment for lp:~robertcarr/mir/implement-client-credentials

> IMO we should refuse the connection earlier (before reading the connect
> message) - in ProtobufSocketCommunicator::on_new_connection(). That ought to
> be simpler and avoid throwing exceptions to tear down a connection that we can
> drop immediately without even registering it with connected_sessions.

seems like an appropriate place to reject the connection