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/proxy.rs

@@ -22,7 +22,7 @@
 
 #![warn(rust_2018_idioms)]
 
-use futures::{future::try_join, FutureExt, StreamExt};
+use futures::{future::try_join, FutureExt};
 use std::{env, error::Error};
 use tokio::{
     io::AsyncReadExt,
@@ -37,9 +37,9 @@ async fn main() -> Result<(), Box<dyn Error>> {
     println!("Listening on: {}", listen_addr);
     println!("Proxying to: {}", server_addr);
 
-    let mut incoming = TcpListener::bind(listen_addr).await?.incoming();
+    let mut listener = TcpListener::bind(listen_addr).await?;
 
-    while let Some(Ok(inbound)) = incoming.next().await {
+    while let Ok((inbound, _)) = listener.accept().await {
         let transfer = transfer(inbound, server_addr.clone()).map(|r| {
             if let Err(e) = r {
                 println!("Failed to transfer; error={}", e);