More SquidUpdate cleanups

More SquidUpdate cleanups

* Remove $wgMaxSquidPurgeTitles; silently discarding URLs is
  the wrong way to limit things. The purge methods already batch,
  which HTCP pushing one at a time. Jobs already batch and have
  $wgJobBackoffThrottling. If code does purge 500 URLs at once
  with no limiting (flooding UDP), then it is already broken.
* Make the HTCP method protected.
* Rename $urlArr field.

Change-Id: I17cced187fe7e93f5a5188022f12202a7746bdb7