[JobQueue] Added aggregate empty/non-empty queue caching.

* The default class is JobQueueAggregatorMemc.
  This essentially has the logic that nextJobDB.php used.
* Also created a JobQueueAggregatorRedis class.
  This is much more efficient and more responsive.
* This can speed up calls to getQueuesWithJobs().
* Removed unused getDefaultQueuesWithJobs() function.

Change-Id: Ifb3c6c881decd643da1b662956ded69db4b39431
Merge "[JobQueue] Added aggregate empty/non-empty queue caching."