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>