More optimizations to User::saveOptions()

* Avoid doing the DELETE for unchanged rows. This method is often
  called when one new preference (e.g. watchlist token) is set.
  These cases will now simple result in an INSERT, even if the
  user already has some preferences set.

bug: 36116
Change-Id: Ib7c558e7c06927875086b86ea719ac1323a7b4a8
Merge "More optimizations to User::saveOptions()"