Added pool counter support for all thumb.php requests

* This can limit how many workers work on a given file at once,
  regardless of which specific thumbnail parameters are requested.
* Also added a BagOStuff::incrWithInit() method per DRY.
* Tweaked the register_shutdown_function() callback to avoid
  File/MediaTransformOutput references, since they may involve
  TempFSFile objects.

Change-Id: Ic01461cb974ab23b179ac3b60cf6de12e36360f6
Merge "Added pool counter support for all thumb.php requests"