Make TlsStream cloneable

`async_std::net::TcpStream` implements `Clone`, which makes it possible to read and write to a socket at the same time.
[Official async-std example](https://github.com/async-rs/async-std/blob/master/examples/tcp-echo.rs)
This library does not implement `Clone` for `TlsStream`, neither does it implement `AsyncRead` or `AsyncWrite` for `&TlsStream`.
How would I rewrite the example above to use TLS when `TlsStream` is not clonable?

Start with the official async-std tcp-echo example linked in the issue and compare its TcpStream ownership with the TlsStream API. Check how TlsStream currently handles AsyncRead and AsyncWrite, then define completion as a TLS version of the example that supports simultaneous reading and writing without ownership workarounds.

Written by the indexing model from the issue text.