libtransport, xmpp: persist MUC names in the DB

- We now store roomlist information in the database (though
  only if your database is postgres, for now...), and use it
  to respond to disco#info queries.
- This should hopefully mitigate any and all issues relating to
  MUCs not having a name -- at least after you've joined them
  at least once (if they weren't in the roomlist)