Refactor of Beanstalkd

Ping @juriansluiman, ready for review.

Once it's ok I'll write the test (or if you want to do it so I can work sooner on SQS :D).

The only thing missing is the watch/ignore/use thing. I don't know where I should put it. In the queue itself ? Or in the SlmQueueBeanstalkd\Service\PheanstalkService ?