dbox: Optimize POP3 MAIL_FETCH_UIDL_BACKEND. We keep track of the highest UID known to have POP3 UIDL in index's header. If saving adds a newer message, it'll also update the header. When fetching UIDL_BACKEND, we can need to check only mails with lower UIDs. There are some race conditions here, but normally UIDLs are set only once during migration so it shouldn't matter.