Added $wgJobBackoffThrottling to replace wmf sleep() hack

* This also adds a Job::workItemCount() method
* Removed unused USE_PRIORITY constant
* A few small cleanups in runJobs.php

Change-Id: Ife9370e488fa63dcd1f702ed98f3b7f26057f10c
Merge "Added $wgJobBackoffThrottling to replace wmf sleep() hack"