Bring back `split` utility as a free fn instead of a method on
`AsyncRead`. This utility wraps the `stream` in an `Arc` and uses mutual
exclusion to ensure correct access.
Additionally, the specialized `split_mut` fn on TcpStream and UdsStream
is promoted to `split`.
A sealed `net::ToSocketAddrs` trait is added. This trait is not intended
to be used by users. Instead, it is an argument to `connect` and `bind`
functions.
The operating system's DNS lookup functionality is used. Blocking
operations are performed on a thread pool in order to avoid blocking the
runtime.
Carl Lerche