Temporary mod to cleanup_notifications.select_notification_ids. Old notifications were hanging around, and as they piled up where bogging down sweep_notifications.select_notifications, which computes a product of notifications and notification_requests that can become quite large. Something's going wrong with the outer join, because querying notification_user_map shows that sent_date is not null for all rows. So we're getting non-matching rows for some reason. Have to figure out why. For now we've simply added a clause that permits deletion of any notification with notif_date greater than 30 days old.
