server: investigate how SOA expiry is handled (esp when 0)

[hannesm]

see https://jpmens.net/2022/01/14/fun-with-the-dns-soa-expire-field/ for some investigations in other name servers... the main purpose of this issue is that we should avoid busy loops (or endless zone retries by immediate expiry). -- bind uses as minimum refresh + retry

the other values of SOA could be guarded for sane as well.