chat-hook: fall back if metadata-store not live
chat-hook: fall back if metadata-store not live

This temporary, upgrade-oriented logic depends on two assumptions:
- If the metadata-store is not running, we are still in the process of
  applying the upgrade.
- If the above is true, all chats are /~host/name, all groups are
  /~/~host/name, and they have a strict one-to-one relation.

Armed with those assumptions, we can deduce groups from chats and vice
versa without depending on the metadata-store, allowing us to carry on
as if it were already running.

urbit/urbit,ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,jfranklin9000/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit