Concurrent vs sequential DNS semantics

[coot]

We noticed the difference between concurrent & sequential semantics. When resolvConcurrent is not set, resolveSequential will loop over errors and will try to return the first non error result, but if resolveConcurrent is set resolveConcurrent will run all queries in parallel doing first-to-finish synchronisation. The difference is that if the first value is an error resolveConcurrent will return the error unlike resolveSequential.

[kazu-yamamoto]

dnsext, a fork of dns, provides the same semantics with an STM magic:
https://github.com/kazu-yamamoto/dnsext/blob/586f42031459fdfc3e753352755d4f7af79ac1d6/dnsext-do53/DNS/Do53/Resolve.hs#L84
If you wish, I can bring it to dns.