Going ahead and adding this field while other DB updates are pending.

Going ahead and adding this field while other DB updates are pending.
Interfaces to use it can be added shortly.

* Add user_editcount field to provide data for heuristics on account use.
  Incremented on edit, with lazy initialization from past revision data.
  Can batch-initialize with maintenance/initEditCount.php (not yet friendly
  to replication environments, this will do all accounts in one query).
* Allow raw SQL subsections in Database::update() SET portion as well as
  for WHERE portion. Handy for increments and such.