Nameserver answering improvements ctd.

Name server:
 - Referral responses.
 - SOA RRSet to response if name not found in zone.
 - Added some helper functions to keep the code more or less clean.

Zone node:
 - cname pointer replaced by union of cname pointer and glue list
   pointer,
 - changed functions accordingly, added getters for cname node and
   glue records.