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-tls/tests/smoke.rs

@@ -524,7 +524,7 @@ async fn client_to_server() {
     drop(env_logger::try_init());
 
     // Create a server listening on a port, then figure out what that port is
-    let srv = t!(TcpListener::bind("127.0.0.1:0").await);
+    let mut srv = t!(TcpListener::bind("127.0.0.1:0").await);
     let addr = t!(srv.local_addr());
 
     let (server_cx, client_cx) = contexts();
@@ -559,7 +559,7 @@ async fn server_to_client() {
     drop(env_logger::try_init());
 
     // Create a server listening on a port, then figure out what that port is
-    let srv = t!(TcpListener::bind("127.0.0.1:0").await);
+    let mut srv = t!(TcpListener::bind("127.0.0.1:0").await);
     let addr = t!(srv.local_addr());
 
     let (server_cx, client_cx) = contexts();
@@ -590,7 +590,7 @@ async fn one_byte_at_a_time() {
     const AMT: usize = 1024;
     drop(env_logger::try_init());
 
-    let srv = t!(TcpListener::bind("127.0.0.1:0").await);
+    let mut srv = t!(TcpListener::bind("127.0.0.1:0").await);
     let addr = t!(srv.local_addr());
 
     let (server_cx, client_cx) = contexts();