Fix consumer signal handler

The callback for pcnt_signal in RpcServerController doesn't exist.
Updated with the right callback.

EDIT: Fix for check signals
We use non-blocking calls ($queue->get()) instead of the blocking call ($queue->consume()), so the signals are not delayed.

Can you explain, why this is needed?

Also I like to leave the consumer without ext-pcntl dependency. If this is really needed, we need to add a check in the constructor, whether or not the extension is available and only call this method after a check for a flag in the consumer.