BE: Schedule the refresh interval from the finish time of the last run
    
    Related: https://pagure.io/SSSD/sssd/issue/4012
    
    Changes scheduling the periodical task so that the next run is started
    relative to the previous run finish time, not start time to protect
    against cases where the refresh would take too long and run practically
    all the time.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>