Add support for storing SAML2 sessions
    
    Store SAML2 session information in a table rather than with the
    user entry so sessions can be persisted past IdP restarts and if
    the user accesses the system via multiple browsers SLO will log
    out all sessions, not just the user session that initiated the
    logout.
    
    https://fedorahosted.org/ipsilon/ticket/90
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>