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-test/src/io.rs

@@ -122,7 +122,7 @@ impl Handle {
     /// The next operation in the mock's script will be to expect a `read` call
     /// and return `buf`.
     pub fn read(&mut self, buf: &[u8]) -> &mut Self {
-        self.tx.try_send(Action::Read(buf.into())).unwrap();
+        self.tx.send(Action::Read(buf.into())).unwrap();
         self
     }
 
@@ -131,7 +131,7 @@ impl Handle {
     /// The next operation in the mock's script will be to expect a `write`
     /// call.
     pub fn write(&mut self, buf: &[u8]) -> &mut Self {
-        self.tx.try_send(Action::Write(buf.into())).unwrap();
+        self.tx.send(Action::Write(buf.into())).unwrap();
         self
     }
 }
@@ -298,7 +298,7 @@ impl AsyncRead for Mock {
                 Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                     if let Some(rem) = self.inner.remaining_wait() {
                         let until = Instant::now() + rem;
-                        self.inner.sleep = Some(time::delay(until));
+                        self.inner.sleep = Some(time::delay_until(until));
                     } else {
                         self.inner.read_wait = Some(cx.waker().clone());
                         return Poll::Pending;
@@ -340,7 +340,7 @@ impl AsyncWrite for Mock {
                 Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                     if let Some(rem) = self.inner.remaining_wait() {
                         let until = Instant::now() + rem;
-                        self.inner.sleep = Some(time::delay(until));
+                        self.inner.sleep = Some(time::delay_until(until));
                     } else {
                         panic!("unexpected WouldBlock");
                     }