grpc in rust bidirectional streaming example using tonic
this example uses tokio's mpsc channels to stream messages between the client and server
constructing a stream from iters that yield also works - but wasn't as neat/intuitive
an initial ping with value 0 is sent from client-> server, the server increments it and sends it back via response stream, which gets sent back to the server via the client req stream and this loops endlessly
cargo run --bin pingpong-server
cargo run --bin pingpong-client