[session] security context directly use transaction object when applicable Most of the security logic have been moved on Transaction anyway.

[session] move context manager logic in the security context manager The `init_security` and `reset_security` method have no other user that the context manager itself. Having dedicated public function is not useful.

[session] hook control context directly use transaction object when applicable Most of the hooks control logic have been moved on Transaction anyway.

[session] move context manager logic in the hook controls context manager The `init_hooks_mode_categories` and `reset_hooks_mode_categories` have no other user that the context manager itself. Having dedicated public function is not useful.

[session] drop the useless set_hooks_mode method It has not external user and trivial code

[session/transaction] move most undo support into transaction A small part that need repo access has not been moved into transaction yet.

[session] simplify mode property We can delegate very thing to transaction utility

[session/transaction] pass the session object to the Transaction __init__ This simplify access to variables. As we are moving more and more logic and states in the Transaction it will greatly helps in future commit.

[session] use a dedicated class to track cnxset We introduce a new CnxSetTracker to track `cnxset` used by Transaction and allows to wait for them. This new class does not use Thread ID not thread joining to work. This allows to use multiple transaction per thread and a transaction in multiple thread. The class itself is totally threadsafe by the Transaction is still not thread safe. The old _threads_in_transaction attribute is dropped in favor of a new logic based on this object. The registration of cnxset used is not done by the Transaction itself. tx.cnset is a property handling the Consistency of its value with the CnxSetTracker instance. Note: The CnxSetTracker instance only track transaction id, not transaction itself, So not reference cycle are created.

[session] cleanup Transaction.cnxset The attribute should always be there. It is None when not cnxset exists.