[server] move connection pooler initialization logic to get_cnxset()
This avoid complex logic in Repository initialization.
[main]
cubicweb.session.secret = %(secret_1)s
cubicweb.auth.authtkt.session.secret = %(secret_2)s
cubicweb.auth.authtkt.persistent.secret = %(secret_3)s
cubicweb.auth.authtkt.session.secure = no
cubicweb.auth.authtkt.persistent.secure = no