Add support for DNS sub-zone delegation.

[GoogleCodeExporter]

Many Direct implementations utilized root domains (ex. mydomain.com) for 
production use, but wish to use sub zones (test.mydomain.com) for 
non-production, sand boxing, testing, etc.  Generally, DNS registrars will 
delegate full responsibility of the root domain to the Direct DNS server 
(needed for CERT RR requests), but the HISP will need to delegate sub zones to 
a different DNS server.

Currently, the Direct DNS server is authoritative only and cannot support the 
configuration above and delegate sub zone requests to another DNS server.  This 
enhancement will allow the DNS server to delegate configured sub zones (done 
via NS records) to a different DNS server.

Original issue reported on code.google.com by [email protected] on 16 Apr 2014 at 10:55