Stricter ZoneName usage.

Stricter ZoneName usage.

While ZoneName is still equivalent to DNSName, this commit turns it into a
separate class (with the same interface as DNSName), and requires conversion
between these classes to be explicit, so that we can recognize the
DNSName/ZoneName boundaries and change them as needs arise.

It is intended for these explicit conversion requirements to be only temporary,
which would allow all these ".operator const DNSName&()" calls to get removed
eventually, once the dust settles and our trust it proper use of ZoneName versus
DNSName is strong enough.