Support for updating subdomain zonefile entries

[kantai]

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]

Moving this over to the subdomain registrar issue tracker: stacks-archive/subdomain-registrar#13

[blockstack-devops]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.