When the CDS/CDNSKEY RRset gets updated, schedule a NOTIFY(CDS) to be
sent to the parental agent. The parental agent is published in the
parent zone as a DSYNC RRset, so first we need to figure out the
parent owner name. This is done by finding the zonecut (querying for
NS RRset until we find a postive answer).
In nsfetch_dsync, we then schedule a zone fetch for the DSYNC record
at <child-labels>._dsync.<parent-labels>. Then we queue the notify
for each target in the DSYNC records that matches the NOTIFY scheme
and CDS RRtype.
Maintain the relationship between the parent and child fetch and when
creating a new child fetch, properly check the resolution loops that
would lead to a new fetch would join one of the parent's fetch contexts.
Scheduling and rescheduling a zonefetch is also similar. Refactor into
zonefetch functions. This also increments and decrements the zone's
internal reference counter in the same module, which may be less
confusing when reading the code.
