Code review comment for ~cjwatson/launchpad:archive-file-history

On second thoughts, we probably shouldn't backfill date_created; NULL is just as good a way to indicate an unknown creation date as the epoch, probably better. Backfilling date_superseded to scheduled_deletion_date minus the stay of execution where applicable does still seem to make sense though, and would allow historical queries to be close enough in most cases (by taking the row with the highest ID that hadn't been superseded at the requested time).