Consider also unavailable presences when dealing with the roster

This is needed for checking `<delay/>` extensions in unavailable presence stanzas.

Another issue remains now: unavailable stanzas received mid-connection. Those have no `<delay/>`, so we have to add it manually when receiving it (actually we could add a <delay/> with the current time to any unavailable stanza we get, if not already present) - although I don't really like this approach either because it modifies stanzas client-side... I don't know what do you think @Flowdalic?