[connection] move hook control logic on Connection
The _hook_control context manager now operate on connection. We have to keep a
specific entry point in Session to ensure Connection object are properly
cleaned up when using session to "manage" Connection live cycle.
Related to #2503918
What's new in CubicWeb 4.0?
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API changes
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* ``RepositorySessionManager.postlogin`` is now called with two arguments,
request and session. And this now happens before the session is linked to the
request.
Deprecated Code Drops
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* The ldapuser source has been dropped. ldapfeed is the only official source
remaining for ldap.
* session.hijack_user mechanism has been dropped.