[rpc module]: SubscriptionSink more user-friendly API for sending items from stream.

[niklasad1]

We have noticed the API for SubscriptionSink is a bit messy to use if you want to await for stream to be sent via the SubscriptionSink if the connection has already been closed it might leak until the stream/fut "resolves" because the stream itself doesn't know anything of the connection state.

For example given that we have the following impl:

impl MyServerTrait for () 
{
	fn subscribe_to_something(&self, mut sink: SubscriptionSink) -> RpcResult<()> {
                // assume this stream lives forever and produces items "rarely"...
		let stream = create_stream();

		let fut = async move {
			stream
				.take_while(|sc| future::ready(sink.send(sc).map_or(false, |_| true)))
				.for_each(|_| future::ready(()))
				.await
		};

	       tokio::spawn(fut);
	}
}

If connection has already this case the tokio task will live until sink.send fails.
To be on the safe-side you would need to something like:

impl MyServerTrait for () {
    fn subscribe_to_something(&self, mut sink: SubscriptionSink) -> RpcResult<()> {
        // assume this stream lives forever and produces items "rarely"...
        let stream = create_stream();

        let fut = async move {
            loop {
                let timeout = tokio::time::sleep(std::time::Duration::from_secs(60));
                tokio::pin!(timeout);

                tokio::select! {
                    Some(item) = stream.next() => {
                        if let Err(e) = sink.send(&item) {
                          break;
                        }
                    },
                    _ = &mut timeout => {
                        if sink.is_closed() {
                            break;
                        }
                    }
                    else => break,
                };
            }
        };

        tokio::spawn(fut);
    }
}