Added support for enqueueable DataUpdates

Added support for enqueueable DataUpdates

* Updates can now declare themselves as having enqueueUpdate()
  as an alternative to doUpdate(). This lets more expensive
  or slave lag producing updates use the job queue if desired.
* Added a $mode flag to DataUpdate::runUpdates() to prefer
  pushing jobs over calling doUpdate().
* Made page deletions defer deletion updates when possible.

Bug: T95501
Change-Id: Ic6f50f92768089ba0fbc223b8d178f5a91512959