Implement the rest of "happy eyeballs" and maybe test the code
We might want to implement more of RFC 8305 ("happy eyeballs") than we did in !382 (merged), as well as writing tests for that code (though I don't think these are priorities).
In particular, we don't implement any of section 4 (interleaving requests using different address families, and sorting the addresses to connect to by some algorithm). We also might want to vary our delay between requests by more than a static amount.