[testlib] use internal_cnx instead of internal_session internal_session is deprecated.

[repository] add an ``internal_cnx`` method to replace ``internal_session`` Accessing the repo through a Session is deprecated. We need an easy replacement for ``internal_session``. This API change was a good occasion to stop disabling integrity hook by default. This is huge source of bug in user-code. related to #2503918

[connection] enforce that a connection must be open to be used The same than for ClientConnection, we ensure the connection is used inside a its context. .. note:: We may rely on that for ClientConnection and remove de dedicated code in Client Connection but I prefer the current explicite and duplicated version for now.

use standalone Connection to Client Connection

[session] add a new_cnx factory Having user-code importing cubicweb.server.session.Connection is inconvenient. We add a simple factory fonction on the session.

[connection] allow simple instantiation of standalone Connection Such connection will automatically pick a connection id. Note, They are not automatically closed on session close. But they will fails to grab new cnxset once the session is closed.

[connection] handle and explicitly life cycle on Connection Like ClientConnection, Connection object need to be explicitly started and stop. They aims to be used as context manager.