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).

tests-integration/tests/process_stdio.rs

@@ -5,7 +5,6 @@ use tokio::process::{Child, Command};
 use tokio_test::assert_ok;
 
 use futures::future::{self, FutureExt};
-use futures::stream::StreamExt;
 use std::env;
 use std::io;
 use std::process::{ExitStatus, Stdio};
@@ -47,9 +46,9 @@ async fn feed_cat(mut cat: Child, n: usize) -> io::Result<ExitStatus> {
         // (i.e. EOF is reached after `n` lines.
         loop {
             let data = reader
-                .next()
+                .next_line()
                 .await
-                .unwrap_or_else(|| Ok(String::new()))
+                .unwrap_or_else(|_| Some(String::new()))
                 .expect("failed to read line");
 
             let num_read = data.len();