Avoid using on_duplicate for station table changes in the update case.

It looks like on_duplicate always reserves and uses a new auto_inc id,
even if there's no conflict and a simple update should occur.

Also avoid some duplicate work regarding lac updates, by grouping cell
entries by lac and avoiding lac updates if all relevant cells are moving.
Avoid on_duplicate in common score update case.
Avoid on_duplicate for adding stats and remove unused incr_stat.

We stopped maintaining the deleted_cell/_wifi stats a while back, after
we started dropping most measures from the online database via the S3
backups.