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).

tokio/tests/rt_common.rs

@@ -41,7 +41,7 @@ rt_test! {
     use tokio::time;
     use tokio_test::{assert_err, assert_ok};
 
-    use futures_util::future::poll_fn;
+    use futures::future::poll_fn;
     use std::future::Future;
     use std::pin::Pin;
     use std::sync::{mpsc, Arc};
@@ -133,12 +133,12 @@ rt_test! {
             let mut txs = (0..ITER)
                 .map(|i| {
                     let (tx, rx) = oneshot::channel();
-                    let mut done_tx = done_tx.clone();
+                    let done_tx = done_tx.clone();
 
                     tokio::spawn(async move {
                         let msg = assert_ok!(rx.await);
                         assert_eq!(i, msg);
-                        assert_ok!(done_tx.try_send(msg));
+                        assert_ok!(done_tx.send(msg));
                     });
 
                     tx