[JobQueue] Cleanups for JobQueueRedis.

* Cleaned up 'server' option to not fragment the pool.
  Also made it actually match the documentation.
* Made it use doGetPeriodicTasks() for job recycling.
* Made it so that other job queue classes can be tested.
* Renamed "redisConf" => "redisConfig".
* Tweaked comments about the "random" order option.

Change-Id: I7823d90010e6bc9d581435c3be92830c5ba68480