DNS renumbering procedure fails if git server is unavailable

it's unclear to me how to renumber a server that only has a record in LDAP (as opposed to something in dns/domains.git as well).

in legacy/trac#33730 (moved), the DNS records were not picked up by nevii, the authoritative nameserver, even after running ud-generate on alberti and ud-replicate on nevii. and indeed, those commands only populate /var/lib/misc/ on nevii, which doesn't touch the full zone loaded by bind.

so this needs clarification and documentation and aaargh.