Add example of detecting client drops in bidirectional streams on server side

[xoac]

closes #781

I think that this example shows also a weakness of tonic. For example in async fn server_streaming_echo() function is impossible (or I do not know how) to get reason why output_stream wasn't able to send message to client.

Motivation

Provide example how to detect client disconnection on server side

Solution

Use tokio::spawn to separate await on input stream from output stream.

[LucioFranco]

Looks like a good start! Left a few comments.

[LucioFranco]

Not following why we want this example as well?

[xoac]

I wanted to show difference between graceful stream end the one when client disconnected for example by broken connection or just crush. This 2 functions could be unified to one.

Here is the output from server part.

EchoServer::server_streaming_echo
	client connected from: Some([::1]:36750)
	client disconnected
EchoServer::Bidirectional_streaming_echo
	stream ended
EchoServer::Bidirectional_streaming_echo
	client disconnected: broken pipe
	stream ended

[xoac]

In other words the difference is on the server side. On the client side I just needed a function that sends request slowly enough that you can write what is printed. I added additional comments to clarify that.

Should I unify this to bidirectional_streaming_echo(client &mut EchoClient<Channel>, num_reqs: usize, throttle: Duration)?

[LucioFranco]

Thanks!