client/finality-grandpa/src/observer: Poll NetworkBridge (#4766)

Patch d4fbb89 reintegrated the periodic neighbor packet worker, by
moving its logic into a `Future` implementation on `NetworkBridge` which
needs to be polled by its upper layer.

Polling by the upper layer was implemented within the `Future`
implementation of the `VoterWork` struct but not within the `Future`
implementation of the `ObserverWork` struct. This patch adds polling of
the `NetworkBridge` to the latter.

client/finality-grandpa/src/observer.rs

@@ -336,10 +336,8 @@ where
 {
 	type Output = Result<(), Error>;
 
-	fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-		let this = Pin::into_inner(self);
-
-		match Future::poll(Pin::new(&mut this.observer), cx) {
+	fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+		match Future::poll(Pin::new(&mut self.observer), cx) {
 			Poll::Pending => {}
 			Poll::Ready(Ok(())) => {
 				// observer commit stream doesn't conclude naturally; this could reasonably be an error.
@@ -351,24 +349,24 @@ where
 			}
 			Poll::Ready(Err(CommandOrError::VoterCommand(command))) => {
 				// some command issued internally
-				this.handle_voter_command(command)?;
+				self.handle_voter_command(command)?;
 				cx.waker().wake_by_ref();
 			}
 		}
 
-		match Stream::poll_next(Pin::new(&mut this.voter_commands_rx), cx) {
+		match Stream::poll_next(Pin::new(&mut self.voter_commands_rx), cx) {
 			Poll::Pending => {}
 			Poll::Ready(None) => {
 				// the `voter_commands_rx` stream should never conclude since it's never closed.
 				return Poll::Ready(Ok(()))
 			}
 			Poll::Ready(Some(command)) => {
 				// some command issued externally
-				this.handle_voter_command(command)?;
+				self.handle_voter_command(command)?;
 				cx.waker().wake_by_ref();
 			}
 		}
 
-		Poll::Pending
+		Future::poll(Pin::new(&mut self.network), cx)
 	}
 }