It is more consistent to have this code closer to the xmpp protocol handling.

It looks like those have not been run for month. A few are failing. They are now commented and wait for a proper fix. (Deferred)

 - when a new contact with higher prio connect, detach session from chat control. Fixes #5021
 - don't re-use a session that was for another sessin

Tests are more than welcome!

prevent a traceback when we get a wrong disco#info reply (a feature without a var attribute). Fixes #5232

Though I consider this a major regression, as using doc-strings is a much
cleaner and better solution. It requires not that big scaffolding to support it,
but some of Gajim developers think otherwise.