When we have authoritative NS records in cache that do not lead to

When we have authoritative NS records in cache that do not lead to
any usable address, wipe the NS records so a query of NS records
to theprant leads to new NS records being inserted into the cache.

Previously the presence of the authoritaivbe records would prevent
that to happen. Queries would *use* the freshly retrieved parent
records, but they would not end up in the cache (the presence of
authoritative NS records prevents that) leading to repeated queries
to the parent.