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.