Mostly mechanical. Creating a `CommonTransport` yields an
`io::Result<_>` now since creating the `DnsConfig` may fail with an
`io::Error` when creating the `ThreadPool`.
The `DnsConfig` `Transport` impl had to be changed slightly:
(a) PR [[1311]] requires some `Send` bounds.
(b) The async block had to be changed to work around lifetime inference
issues which resulted in an "one type is more general than the other"
error.
[1311]: https://github.com/libp2p/rust-libp2p/pull/1311
