Use Async::new_nonblocking to avoid redundant socket configuration

[BigWingBeat]

This PR replaces Quinn's use of Async::new with Async::new_nonblocking, a constructor introduced in version 2.0 of async_io, which assumes the socket has already been put into non-blocking mode. For Quinn this is indeed the case, as quinn_udp configures the socket as non-blocking already.

As for the Tokio runtime implementation, that uses UdpSocket::from_std, which already makes this assumption.

[djc]

Makes sense, thanks! I am curious how this helps your use case?

[BigWingBeat]

This wasn't motivated by a particular use-case, it's just a redundancy with a trival fix I noticed while looking at the code.

[Ralith]

Thanks!