`dns-client.unix` should integrate a way to communicate to a DNS server via `tls`

[dinosaure]

Currently, this is the diff between dns-client.lwt and dns-client.unix. I know that we missed something like tls.unix/tls-unix but I did one, do you think that it's interesting or should I do something on my on way?

[hannesm]

I guess it boils down to effort in maintenance. in some projects, I avoid lwt, so in general I'm fine to have a tls.unix implementations. This could be easily merged and released -- and once effects are out, we may be able to remove all unix/lwt/async/mirage and be fine with only a single effectful layer :)