[JobQueue] Improved job recycle rate for small queues.

[JobQueue] Improved job recycle rate for small queues.

* Make the recycling a bit more periodic rather than based on
  how often pop() gets called essentially. This works better if
  a queue does not have jobs inserted very often.

Change-Id: I64fbc8afbb1cf096717ba4bfc6fe7b7715abdb72