Basic classes (evaluated in following order):
* NORMAL - positive answer
* ERROR - rcode is not NXDOMAIN nor NOERROR
* NXDOMAIN - rcode is NXDOMAIN
* EMPTY - response doesn't contain any answers
* LARGE - packet size exceeded threshold (currently 1k)
* WILDCARD - answering from a wildcard

Reason behind not selectively classifying popular types like
DNSKEY, RRSIG or ANY is that any type could be exploited,
depending on the contents of the zone.

refs #2136