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/sync_errors.rs

@@ -6,11 +6,8 @@ fn is_error<T: ::std::error::Error + Send + Sync>() {}
 fn mpsc_error_bound() {
     use tokio::sync::mpsc::error;
 
-    is_error::<error::SendError>();
+    is_error::<error::SendError<()>>();
     is_error::<error::TrySendError<()>>();
-    is_error::<error::UnboundedRecvError>();
-    is_error::<error::UnboundedSendError>();
-    is_error::<error::UnboundedTrySendError<()>>();
 }
 
 #[test]