Add try_clone() for TlsStream (same as the TcpStream

[WalterYongtaoWang]

TcpStream has this try_cone() method, so you can read/write the stream in different threads: https://doc.rust-lang.org/std/net/struct.TcpStream.html#method.try_clone

Please add it to TlsStream so we can do the same.

[sfackler]

The internal TLS implementations do not support that operation.