Delete a zone from the list of failed slave-checks on incoming NOTIFY and fix d_lock...

Delete a zone from the list of failed slave-checks on incoming NOTIFY and fix d_lock handling

- If the master is not available, PDNS uses an incrmental backoff for SOA-checks to avoid
constant SOA checks on the failed master server. If the master server comes back to life
and sends a NOTIFY, the NOTIFY is ignored due to the backoff. This patch removes the zone
from the list of failed slave-checks to allow immediate checking.

- Debug-Log if a slave-check was skipped due to incremental backoff feature

- Fix d_lock handling according to comments on https://github.com/PowerDNS/pdns/pull/6819

- Be more verbose about the consequences of a failed SOA check