[session] Drop no more necessary Session.sessionid and Connection.connectionid

[repo] Kill repo._sessions and with it the need to have a looping task to clean it up, and with that all the session's timestamping machinery.

[repo] Stop closing session The only point in "closing" session is to call the `session_close` event. Since I'm not aware of any application relying on it, I think this is too costly (in term of code) to maintain and propose to drop it.

[views] Stop giving stats / debug information relying on _sessions This is already broken when using pyramid anyway.

[session] Stop relying on _sessions to synchronize living session by doing what we currently do in pyramid. Also, attempt to synchronize CWUser's class sounds like a dark corner case handling that should be removed in favor of short lived user objects (beside it shouldn't occur except during migration).

[test] Drop useless access to repo._sessions test still succeed afterwards.

[repo] Fix flake8 errors in server/__init__.py

[repo] Drop no more used ShuttingDown exception

[repo] Drop _get_session method it's not actually necessary but holds underlying _sessions dict handling. Drop backward compat relying on it (which IMO should not be a problem). Drop import of unused QueryError along the way.

[test] Drop call to CubicWebConfiguration.load_cwctl_plugins in unittest_cwctl.py This is no longer need now that we call load_available_configurations() in the "list" command (see 5e7282bdf140).