watchdog: cope with time shift
    
    When a time is changed into the past during sssd runtime
    (e.g. on boot during time correction), it is possible that
    we never hit watchdog tevent timer since it is based on
    system time.
    
    This patch adds a past-time shift detection mechanism. If a time
    shift is detected we restart watchdog.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3154
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>