Extract job hydration to the queue instance

Please review @basz and @bakura10. I'd like feedback from both whether this is heading in the proper direction.

My problem: the job itself is serializing the content but the queue unserializes and instantiates the job via the job plugin manager. To me, these are "kind of" the same responsibilities but executed in two different layers. I'd like to fix this. For now, I choose to use the queue as it has knowledge how to instantiate the job by a service name. 

In a next step, we could make a separate JobHydrator class which converts the job instance to and from strings (using json and/or serialization). I created a branch for SlmQueueBeanstalkd to see how that works out. For me, it feels like the right step into simpler usage of adapter classes.