[JobQueue] Added a JobQueueRedis class.

* The Job class now includes an extra "metadata" field now for usage
  by the queue classes. This is useful for stores that lack IDs.
* A RedisConnectionPool class was added for reusability.

Change-Id: Ie72536240e27fe1caf11096cdc4c85dfa821d181
Merge "[JobQueue] Added a JobQueueRedis class."