Support for updating subdomain zonefile entries

[jcnelson]

Currently, our subdomain registrar only handles registration requests. It should additionally handle updating those entries.

The resolver supports parsing and verifying these updates, but an endpoint needs to be added to the registrar (and verification code for the registrar, so that it doesn't add invalid entries to it's zonefiles, even if the resolvers will ultimately ignore them).

[jcnelson]

It occurs to me that this is going to be really important once we support migrating data from one Gaia hub to another.