[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

[application/connect] simplify connection logic ``application.connect`` now either sets a full featured ``DBAPISession`` to the ``WebRequest`` object or raises ``AuthenticationError``. The creation and usage of a fake DBAPISession is now handled by ``main_handle_request`` when needed. This means that fake DBAPISession are no longer tracked by the session manager and that user are not given anyway to retrieve them for a later request. This fake DBAPISession is still passed to ``core_handle`` because multiple cubes like registration or forgotten password need this behavior. We would like to get ride of it in the future. This clarification of the connection API greatly simplifies ``DBAPISession`` retrieval//creation process opening the way to improvements in this area. Related to #2503918