A CAA record with accounturi covers every name under the domain, so another
ACME client there, like certbot on a web server, is refused at its next
renewal. Nothing warned about it and it could be weeks before it showed.
New domains now have to ask for CAA. The field description says why, and
domains that already publish it keep it.
The DNS-PERSIST-01 validation record was only built as part of CAA, so
without this a persist provider on a domain not publishing CAA could never
validate. It's now published either way for those providers.
Fixes#292