netcheck: split DNS lookup for IPv6 and IPv4

[flub]

We currently try to lookup both A and AAAA records in DNS for probes. But our probes, excluding https which handles it's own DNS, are split per protocol anyway. We should instead split the DNS lookup so that each probe can have results faster.

Right now we can see sometimes that the first probe of a probeset needing DNS lookups is rather slow, and the 2nd probe in the set starts (which then gets a cached DNS result and is fast, but usually still slower than the first probe). I'm not sure that can really be improved much, but any speedup in DNS results will help.