Allow specifying a primary server and retrieve the zone's content.
Add tests and an Expired bool to zone struct, to stop server zones
that are expired. The zone is retrieved on Startup, no updates of
changed content are done. We also don't respond to notifies yet.
Full implementation, DNS (and in the future DNSSEC). Returns answer in a
hopefully standards compliant way.
Testing with my miek.nl zone are included as well.
This should correctly handle nodata, nxdomain and cnames.