This can be registered with the raiden AlarmTask.
        If `EchoNode.stop()` is called, it will give the return signal to be removed from
        the AlarmTask callbacks.

def echo_node_alarm_callback(self, block_number):
        """ This can be registered with the raiden AlarmTask.
        If `EchoNode.stop()` is called, it will give the return signal to be removed from
        the AlarmTask callbacks.
        """
        if not self.ready.is_set():
            self.ready.set()
        log.debug('echo_node callback', block_number=block_number)
        if self.stop_signal is not None:
            return REMOVE_CALLBACK
        else:
            self.greenlets.add(gevent.spawn(self.poll_all_received_events))
            return True