Report not connected/authenticated when resumeable

Since c81cd3456146d788080923d0acc2ac7371f82c97 we can report not
connected/authenticated when the client is disconnected but the stream
could be resumed. The previous behavior caused some confusion among
the users.
Log a warning if the server didn't report SASL mechanisms

and return the empty list.
Set smWasEnableAtLeastOnce before sync point

to allow users to add stanza acknowledged listeners in
ConnectionListener.authenticated().