Move user_editcount updates to a mergeable deferred update

This should reduce excess contention and lock timeouts.
Previously, it used a pre-commit hook which ran just before the
end of the DB transaction round.

Also removed unused User::incEditCountImmediate() method.

Bug: T202715
Depends-on: I6d239a5ea286afb10d9e317b2ee1436de60f7e4f
Depends-on: I0ad3d17107efc7b0e59f1dd54d5733cd1572a2b7
Change-Id: I0d6d7ddd91bbb21995142808248d162e05696d47