cubicweb #4151635 [sessions] have "volatile" sessions [rejected]

A volatile session:

* has the life-time of a web request
* is not known from repo._sessions or sessionmanager._sessions

Hence the associated memory is freed at the end of the request life time.
This will help applications using many "signed requests", which can result in tens of thousands of un-reclaimed session objects that ultimately bog the cubicweb instance down to the oom killer.

done in3.20.0
load left0.000
closed by<not specified>
patch[sessions] introduce the `extent` notion for session. [rejected][repository] provide a .new_session entry point [applied]