Various fixes and simplifications to RefreshLinksJob::runTitle()

Various fixes and simplifications to RefreshLinksJob::runTitle()

* Remove logic for saving slow-to-render parser output. This has
  not worked ever since DerivedPageDataUpdater was introduced.
* Make the logic to use cached output actually work. This was
  also broken since DerivedPageDataUpdater was added. In order
  to pass the output, add a known-revision-output parameter
  to both WikiPage::doSecondaryUpdates() and
  DerivedPageDataUpdater::prepareUpdate().
* Also factored out some helper methods from runForTitle() in
  RefreshLinksJob to make it more readable and avoid the need
  for multiple transaction round commit calls. This makes the
  case of multiple-title jobs less likely to break again.
* Make use of RefreshLinksJob::runForTitle() return value.
* Add unit tests for multiple-title job case.

Change-Id: I0cd13c424a87653b5a7253c42cd48fe43befd692