Improved partitioning scheme for refreshLinks jobs

* The changes refreshLinks to handle both per-title (leaf) and backlink jobs.
  The base job now splits into some leaf jobs and a remaining partition job.
  The partition job does the same until there are only a small number
  of backlinks in the remaining range (so only leaf jobs are added).
  Since the leaf jobs are pushed first, this works well for FIFO queues
  to avoid bloating the queue. This also improves per-title job
  de-duplication, which isQueueDeprioritized() pretty much killed.
* The refreshLinks2 class is no longer used for new jobs.
* Fix process cache bug with JobQueueGroup::push with empty arrays.
* This adds a BacklinksJobUtils with helper functions for partitioning.
* RefreshLinksJob jobs now have a simple version parameter.
* Also moved refreshLinks2Job to its own file.

Change-Id: Id378d47df17248ae02938d5a54ef7ecd29efadbd