transport_service: Improve logs and move code from tokio::select macro (

#254)

Tiny PR to enhance the transport service logs with protocol and
endpoints.

While at it, have refactored the code to move away from the tokio select
and use dedicated fns.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

src/protocol/transport_service.rs

@@ -389,7 +389,10 @@ impl Stream for TransportService {
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         while let Poll::Ready(event) = self.rx.poll_recv(cx) {
             match event {
-                None => return Poll::Ready(None),
+                None => {
+                    tracing::warn!(target: LOG_TARGET, "transport service closed");
+                    return Poll::Ready(None);
+                }
                 Some(InnerTransportEvent::ConnectionEstablished {
                     peer,
                     endpoint,