UtpStream is not "Send"

[jason-ni]

Hi,

I write an example of sending message through a utp stream: jason-ni@76b93ec

However, it can not be compiled. It seems the "Send" marker is not sufficient to allow spawning a future with UtpStream. I tried to add the "Sync" marker. Then it can be compiled and run as expected. But I don't think that's the correct fix. My simple code may not be complicated enough to trigger some race condition.

Another issue is the UtpStream is hard to be split into read half and write half like tokio's TcpStream. Splitting to 2 separated halves is critical to implement a full-duplex proxy, as I understand(tokio-rs/tokio#1108).

Maybe we need to add another layer of indirection by using channel. How do you think?