Extracted a new WorkerQueueList class out of Rescue::Worker

Currently Rescue::Worker is responsible for several things, this is an
attempt to move the queue management logic into a seperate class.

I left Rescue::Worker#queues method in due to legacy tests failing if I
removed it. Instead I let it delegate its work to the new
WorkerQueueList class. But if no external class is calling it then it should be
safe to remove it.

* Extracted @queues from Rescue::Worker into a WorkerQueueList class
* Modified Rescue::Worker to use the the new WorkerQueueList class.
Merge pull request #983 from einarj/queue_list

Extracted a new WorkerQueueList class out of Rescue::Worker