[session] deprecate all Connection related method on session

[session] drop dead _current_cnx_id Not used anymore for a few commit.

[session] privatise get_cnx and close_cnx The only user, repoapi now use ``new_cnx``

[doc] add documentation for the new API in test

[testlib] add an default testcase.adminaccess (and use it for default session) This adminaccess is the new offical way to get connection, and request on a repo.

[testlib] introduce a RepoAccess class to easily create connection and request Each RepoAccess hold a session for a user and three helper function to help create Connection, ClientConnection of WebRequest related to this session. related to #2920299

[repoapi] make ClientConnection.__enter__ return self This allow the standard idiom:: with repoapi.connect(repo, login='babar', passwork='elephant') as cnx: cnx.execute(…)

[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