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.