More tweaks to job backoff code

* Replace one time() call with microtime() in syncBackoffDeltas().
  Also moved the call down slightly to not count flock() delay.
* Moved read-only case logic into syncBackoffDeltas().
* Moved $backoffExpireFunc logic into syncBackoffDeltas().
* Tightened the syncBackoffDeltas() checks around pop() for
  better accuracy.

Change-Id: Ifed3d24ba62277c0e0f52cdc1051990a590be18a
Merge "More tweaks to job backoff code"