Roman Borschel cd15bc9c62
[libp2p-dns] Use trust-dns-resolver (with either async-std or tokio). Remove thread pool. (#1927)
* [libp2p-dns] Use trust-dns-resolver.

Use the `trust-dns-resolver` library for DNS resolution,
thereby removing current use of the thread pool.

Since `trust-dns-resolver` and related crates already
provide support for both `async-std` and `tokio`, we
make use of that here in our own feature flags.

Since `trust-dns-resolver` provides many useful
configuration options and error detail, central
types of `trust-dns-resolver` like `ResolverConfig`,
`ResolverOpts` and `ResolveError` are re-exposed
in the API of `libp2p-dns`. Full encapsulation
does not seem preferable in this case.

* Cleanup

* Fix two intra-doc links.

* Simplify slightly.

* Incorporate review feedback.

* Remove git dependency and fix example.

* Update version and changelogs.
2021-03-16 11:48:48 +01:00
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