Limit futures dependency to Stream via feature flag (#1774)

In an effort to reach API stability, the `tokio` crate is shedding its
_public_ dependencies on crates that are either a) do not provide a
stable (1.0+) release with longevity guarantees or b) match the `tokio`
release cadence. Of course, implementing `std` traits fits the
requirements.

The on exception, for now, is the `Stream` trait found in `futures_core`.
It is expected that this trait will not change much and be moved into `std.
Since Tokio is not yet going reaching 1.0, I feel that it is acceptable to maintain
a dependency on this trait given how foundational it is.

Since the `Stream` implementation is optional, types that are logically
streams provide `async fn next_*` functions to obtain the next value.
Avoiding the `next()` name prevents fn conflicts with `StreamExt::next()`.

Additionally, some misc cleanup is also done:

- `tokio::io::io` -> `tokio::io::util`.
- `delay` -> `delay_until`.
- `Timeout::new` -> `timeout(...)`.
- `signal::ctrl_c()` returns a future instead of a stream.
- `{tcp,unix}::Incoming` is removed (due to lack of `Stream` trait).
- `time::Throttle` is removed (due to lack of `Stream` trait).
-  Fix: `mpsc::UnboundedSender::send(&self)` (no more conflict with `Sink` fns).

examples/chat.rs

@@ -116,18 +116,12 @@ impl Shared {
 
     /// Send a `LineCodec` encoded message to every peer, except
     /// for the sender.
-    async fn broadcast(
-        &mut self,
-        sender: SocketAddr,
-        message: &str,
-    ) -> Result<(), mpsc::error::UnboundedSendError> {
+    async fn broadcast(&mut self, sender: SocketAddr, message: &str) {
         for peer in self.peers.iter_mut() {
             if *peer.0 != sender {
-                peer.1.send(message.into()).await?;
+                let _ = peer.1.send(message.into());
             }
         }
-
-        Ok(())
     }
 }
 
@@ -218,7 +212,7 @@ async fn process(
         let mut state = state.lock().await;
         let msg = format!("{} has joined the chat", username);
         println!("{}", msg);
-        state.broadcast(addr, &msg).await?;
+        state.broadcast(addr, &msg).await;
     }
 
     // Process incoming messages until our stream is exhausted by a disconnect.
@@ -230,7 +224,7 @@ async fn process(
                 let mut state = state.lock().await;
                 let msg = format!("{}: {}", username, msg);
 
-                state.broadcast(addr, &msg).await?;
+                state.broadcast(addr, &msg).await;
             }
             // A message was received from a peer. Send it to the
             // current user.
@@ -254,7 +248,7 @@ async fn process(
 
         let msg = format!("{} has left the chat", username);
         println!("{}", msg);
-        state.broadcast(addr, &msg).await?;
+        state.broadcast(addr, &msg).await;
     }
 
     Ok(())