rename server.session.transaction into server.session.connection The ongoing rework of the API to access the database include the splitting of Session in two objects: The ``Session`` object will only hold credential (user) and global session data. A new object will publicly emerge to handle a database access. This object is named ``Connection`` since that the way database accessors with the same property are named in other system. So we renamed the ``Transaction`` object into ``Connection``. The ``Transaction`` object have already grown in the direction of something directly usable by generic code, but the name ``Transaction`` is ill suited as such object can be used for multiple database transaction in a row. Related to #2503918

[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