Fix dnslink lookup

[dvc94ch]

Previously domain was used but that is tokio-0.1 library. Now domain seems to have upgraded to latest tokio in https://github.com/NLnetLabs/domain.

Our needs for the DNS are:

• stub resolving
• TXT records to get dnslink 1 2

rust-libp2p should already handle resolving names in /dns4 and /dns6 multiaddrs through threadpooled (stub?) resolver (haven't checked which library).

Being able to resolve dnslinks is the desired outcome. The async-std dependency seems a bit difficult. Alternatives on tokio would include trust-dns. Even if we decide to add back tokio support, we should be considering the what ever way libp2p is using as something we could leave for async-std.

[koivunej]

I think this is now fixed with some of ljedrz latest dep update PRs.

[ljedrz]

Ah yeah, the related tests pass (when enabled with tokio) after cb0fa7b.

[koivunej]

And now we are fully tokio so this is always enabled.