[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.

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−# -*- coding: utf-8 -*-
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−# copyright %(year)s %(author)s, all rights reserved.
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−# contact %(author-web-site)s -- mailto:%(author-email)s
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−#
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−%(long-license)s
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−"""%(distname)s schema"""