Change SAML2 sessions backend to use Store API
    
    The basic session API remains the same, just replace
    the calls to pull data out of the user session to
    instead pull from the database.
    
    The per-session logout state is now a constant rather than
    being a member of either the logged_in or logging_out
    dictionaries.
    
    https://fedorahosted.org/ipsilon/ticket/90
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>