DNS records can now be a A record or a CNAME record. All we care
about is them resolving to the public IP of the server somehow.
The main reason for this change is that altDomain is migrated into
domains table and the DNS propagation checks have to work after that.
(previously, the 'altDomain' was a signal for a CNAME check which now
cannot be done post-migration).
In the future, we can make this more sophisticated to instead maybe
do a well-known URI query. That way it will work even if there is
some proxy like Cloudflare in the middle.
Fixes#503
