Extend examples/async_echo.rs to support concurrent requests serving for benchmarking

[Carter Li]

I know little about Rust, but it seems that

rio/examples/async_echo.rs

Line 23 in 4c6baca

 if let Err(_) = proxy(&ring, &stream, &stream).await { 

blocks future connections before the current client is disconnected, which generally forbids concurrent requests serving. Only one connection is allowed at the same time.

Related project: https://github.com/haraldh/rust_echo_bench

Any thoughts?