[application] call req.set_session in application.main_handle_request The Session handling chain is no more responsible for calling req.set_session. It just returns a valid session and lets the caller link it to the Request. This opens the way to explicitly creating and closing a connection/transaction in ``application.main_handle_request``. Related to #2503918

[session-handler] use session directly to update last usage We don't really need the WebRequest for that. Not using the WebRequest to access the cubicweb repository here will allow a delayed set_session. Related to #2503918