[connection] transparent cnx_set handling
Connection object while take cares of there cnxset themself (as dbapi connection
does).
The ``set_cnxset`` and ``free_cnxset`` operation are still available for
backward compatibility purpose. The ``_auto_free_cnx_set`` is introduced to
handle mixed usage.
A new context manager ``connection.ensure_cnx_set`` is added for code that
access ``cnx.cnxset`` directly and are not wrapped in any specific
``Connection`` method.
A ``_with_cnx_set`` decorator is used on all Connection method that need a
cnxset.
What's new in CubicWeb 4.0?
============================
Behavior Changes
----------------
* The anonymous property of Session and Connection are now computed from the
related user login. If it match the ``anonymous-user`` in the config the
connection is anonymous. Beware that the ``anonymous-user`` config is web
specific. Therefore, no session may be anonymous in repository only setup.
API changes
-----------
* ``RepositorySessionManager.postlogin`` is now called with two arguments,
request and session. And this now happens before the session is linked to the
request.
* ``SessionManager`` and ``AuthenticationManager`` now take a repo object at
initialization time instead of a vreg.
* The ``async`` argument of ``_cw.call_service`` have been dropped. All call are
now synchronous. The zmq notification bus looks like a good replacement for
most async usecase.
* ``repo.stats()`` is now deprecated. The same information are available through
a service (``_cw.call_service('repo_stats')``)
* ``repo.gc_stats()`` is now deprecated. The same information are available through
a service (``_cw.call_service('repo_gc_stats')``)
* ``request.set_session`` no longer takes an optional ``user`` argument.
* CubicwebTC does not have repo and cnx as class attributes anymore. They are
standard instance attributes. ``set_cnx`` and ``_init_repo`` class methods
become instance methods.
Deprecated Code Drops
----------------------
* The ldapuser source has been dropped. ldapfeed is the only official source
remaining for ldap.
* session.hijack_user mechanism has been dropped.