Delete stale files

The mailbox should delete files that were uploaded a long time ago and will no longer be decryptable by their recipients.

The definition of "a long time ago" should depend on the key rotation period, which is max latency + max clock difference, where max clock difference is 24 hours. We're considering a max latency of 14 days for mailboxes, in which case the rotation period would be 15 days and files could safely be deleted after 30 days.