Signals not handled when the queue is empty

I can offer quick solution related to videlalvaro/RabbitMqBundle#178
It couldn't slowdown worker too much.
i used name of constant from RabbitmqBundle https://github.com/videlalvaro/RabbitMqBundle/blob/master/RabbitMq/BaseConsumer.php#L50
btw it should works "as is" and better if signals are used