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.
can we actually make it the other way around, call it `AMQP_WITH_SIGNALS`, which must be set to `true` in order to enable the `pcntl_signal_dispatch`. I think this will give the feeling that if I keep my code "as is", then it will work as before.