move hooks activation control on session object, so we can have a per transaction control. Added a new `hooks_control` context manager for usual modification of hooks activation.
# cube's specific schema
"""
:organization: Logilab
:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""