Read responses in parallel with writing request when pipelining

[link2xt]

See comment at https://github.com/async-email/async-smtp/pull/57/files#r1090076853

[link2xt]

From RFC 2920:

Client SMTP implementations MAY elect to operate in a nonblocking
fashion, processing server responses immediately upon receipt, even
if there is still data pending transmission from the client's
previous TCP send operation. If nonblocking operation is not
supported, however, client SMTP implementations MUST also check the
TCP window size and make sure that each group of commands fits
entirely within the window. The window size is usually, but not
always, 4K octets. Failure to perform this check can lead to
deadlock conditions.

[link2xt]

Spawning a task to write requests, while reading responses in the main task, may work, but splitting generic stream into read and write part is complicated. It's not easy to do even for TcpStream in tokio currently: tokio-rs/tokio-core#198
I know it should be as easy as copying the file descriptor of the underlying TCP socket, but async TcpStream is currently not cloneable, unlike std::io one.