add incremental slave-check backoff also for failed AXFR due to master problems

add incremental slave-check backoff also for failed AXFR due to master problems

If SOA-checks against a master with failing AXFR still succeed, we would
constantly try to AXFR the zone. To avoid this, we add the zone to the list
of failed slave-checks to suspend slave-checks (and subsequent AXFR) for
this zone for some time.