Answer dns record queries

Dns queries for record types are now answered by mdns.  The answers are
obtained from storage and returned.  Currently REFUSED is returned for
SOA.  Once the SOA record type is persisted to storage similar to other
record types, SOA queries would succeed.

Change-Id: I21a766d0564a71b1eb2b7ae0163cb5c786eca48c
Implements: blueprint mdns-designate-mdns-functional