Clean up SM callbacks when not proceeding with SM.
Clean up SM callbacks when not proceeding with SM.

The SM stream feature is offered and processed twice during stream
startup:

1) Before binding, so you could resume session if still have a token
2) After binding, to enable SM

Callbacks for server responses were registered during 1), but if there
was no token for resumption, the feature callback was called
immediately. However, that meant during processing of 2) when SM was
enabled and the server responded, the feature callback from 1) was being
called again because the server response handlers were still there.

All exits from SM feature processing now result in releasing the SM
server response group.

Fixes #106

otalk/stanza.io,flavionegrao/stanza.io,otalk/stanza.io,flavionegrao/stanza.io,Palid/stanza.io,Palid/stanza.io,legastero/stanza.io,legastero/stanza.io