[JobQueue] Added support for delayed jobs with JobQueueRedis.

* The queue can handle delaying jobs until a given timestamp is reached.
* Added Job::getReleaseTimestamp() to let jobs specifiy delay amounts.
* Added a "checkDelay" option and a supportsDelayedJobs() function to JobQueue.
  There are also getDelayedCount() and getAllDelayedJobs() functions.
* Simplified a bit of code in doBatchPush() and pushBlobs().
* Improved the logic in redisEval().

Change-Id: I40b3e3438e659f6844bdbdd5e9d3ccc6c4dc82b2