[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.

if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',
           ask_confirm=False):
    rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',
        ask_confirm=False)