[migration] fix bug in `CWAttributeAddOp.revertprecommit_event` The `rdefdef` attribute is set during the operation execution. In some the operation crash before this assignment. In such case `revertprecommit` raise an attribute error crashing the whole process and shadowing the original error. This changeset detect and Avoid this situation.

[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.