[documentation] describe repoapi and web side change. Short version explaining what object replace what and that BC existes for a few version.

[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.

[sesion] distinction between Connection handled by the session and other. Not mixing the new and backward compat approach seems a good idea. Let's enforce it.

[session] replace _clear_thread_storage with close_cnx There is not good reason to keep two distinct method.

[session] explicitly take Connection object in close_cnx Now that ClientConnection explicitly reference and use the Connection object we do not need to use connectionid here. We can safely change this signature, ClientConnection is the only use of close_cnx for now.

[session] drop the Session._clear_cnx_storage method It is just cnx.clear() now.