correct new_measures bookkeeping for the cell backfill case
Don't update the creation time on existing cell blacklist records.

The code in `mark_moving_wifis` might look confusing here as it does:
`on_duplicate='created=created'` which is a no-op. It's only there as
on_duplicate requires at least one column clause. But this one only says:
update created to what is already in the database.