[JobQueue] Added support for retrying jobs when runners die.

[JobQueue] Added support for retrying jobs when runners die.

* This adds a new 'claimTTL' setting which recycles jobs into the queue
  if they have been claimed for longer than that duration (in seconds).
* This also purges jobs for such cases when job retries are not enabled.
* This adds a new job_attempts column and adds an index to improve the
  query and semantics for FIFO ordered queues.

Change-Id: Idb6217a694d86a4d6fc881768deed424628f015d