Remove expired SAML2 sessions
    
    Run a cherrypy background task to sift through the sessions
    database and find expired entries and remove them.
    
    From my testing if a previous execution of the background task
    is still executing when the next one is scheduled to run, it will
    skip it. In other words, you can't end up with multiple expirations
    running at the same time.
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>