[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.