Fix Subscription cancelation when never awaiting

`StreamExt::forward` will keep polling a ready `Stream` in a loop. If the
`Stream` is always ready, the `poll` method of `Forward` effectively
blocks (see rust-lang/futures-rs#2552).

The fix consists in manually implementing a simpler version of `Forward`.

futures/src/subscription/tracker.rs

@@ -1,6 +1,9 @@
 use crate::{BoxFuture, MaybeSend, Subscription};
 
-use futures::{channel::mpsc, sink::Sink};
+use futures::{
+    channel::mpsc,
+    sink::{Sink, SinkExt},
+};
 use std::{collections::HashMap, marker::PhantomData};
 
 /// A registry of subscription streams.
@@ -85,19 +88,30 @@ where
                 continue;
             }
 
-            let (cancel, cancelled) = futures::channel::oneshot::channel();
+            let (cancel, mut canceled) = futures::channel::oneshot::channel();
 
             // TODO: Use bus if/when it supports async
             let (event_sender, event_receiver) =
                 futures::channel::mpsc::channel(100);
 
-            let stream = recipe.stream(event_receiver.boxed());
-
-            let future = futures::future::select(
-                cancelled,
-                stream.map(Ok).forward(receiver.clone()),
-            )
-            .map(|_| ());
+            let mut receiver = receiver.clone();
+            let mut stream = recipe.stream(event_receiver.boxed());
+
+            let future = async move {
+                loop {
+                    let select =
+                        futures::future::select(&mut canceled, stream.next());
+
+                    match select.await {
+                        futures::future::Either::Left(_)
+                        | futures::future::Either::Right((None, _)) => break,
+                        futures::future::Either::Right((Some(message), _)) => {
+                            let _ = receiver.send(message).await;
+                        }
+                    }
+                }
+            }
+            .boxed();
 
             let _ = self.subscriptions.insert(
                 id,