OStatus/FeedSub: tweaked PuSH feed garbage collection so other plugins can declare...

OStatus/FeedSub: tweaked PuSH feed garbage collection so other plugins can declare usage of a low-level feed or an OStatus profile besides profile subscriptions & group memberships.
SubMirror: redid add-mirror frontend to accept a feed URL, then pass that on to OStatus, instead of pulling from your subscriptions.
Profile: tweaked subscriberCount() so it doesn't subtract 1 for foreign profiles who aren't subscribed to themselves; instead excludes the self-subscription in the count query.
Memcached_DataObject: tweak to avoid extra error spew in the DB error raising

Work in progress: tweaking feedsub garbage collection so we can count other uses