Register a callback for received messages (that we didn't initiate)

        :param func callback: A function which should be called when data is received

def on_recv(self, callback):
        '''
        Register a callback for received messages (that we didn't initiate)

        :param func callback: A function which should be called when data is received
        '''
        if callback is None:
            return self.stream.on_recv(None)

        @tornado.gen.coroutine
        def wrap_callback(messages):
            payload = yield self._decode_messages(messages)
            if payload is not None:
                callback(payload)
        return self.stream.on_recv(wrap_callback)