Remove User::$mEditCount and internal caching
UserEditTracker now returns null for anonymous users
instead of throwing an exception.
Added new UserEditTracker::setCachedUserEditCount()
method (internal) to handle the user_editcount field
in User::loadFromRow()'s row. This now means that
the creation of one User object can set the cached
value for any other object for the same underlying user
- the alternative (just ignoring the value) would mean
that an extra database read might be needed. This is
only used for users created with a row that includes
the user id - we silently discard the value if
the row doesn't have a user id (or the user id is 0)
to avoid issues with UserEditTracker trying to fetch
the user id and potentially triggering another load.
A follow-up will simplify the UserEditCountUpdate class
to remove storing full UserIdentity objects now that
only the id is needed.