libtransport, xmpp: attempt to reconnect to pg, fix presences

- Oops we forgot to run clang-format earlier, so now a bunch of unrelated
  stuff got clang-formatted into this change. :/
- Building on the already-super-hacky Postgres database backend, we now
  attempt to reconnect to the database every time there's an error.
  This is bad, but better than it just dropping connection and then
  failing all database operations afterwards...
  Ideally libpqxx would handle this, but of course it doesn't.
- The Buddy code did some super crazy crap around sending out multiple
  presences for each buddy???? I don't get it, so it's been replaced
  with something far saner.
- The RosterManager wouldn't send presence for contacts that weren't online.
  This is stupid, and it doesn't do that any more.
- Additionally, buddies would only send their photo hash when they were
  online, which is also stupid (and not spec-compliant). Fixing this
  means we actually have avatars for all of the buddies in the roster,
  which is nice!