Implement throttle combinator (#736)

Throttle down a stream by enforcing a fixed delay between items.

tokio-timer/tests/throttle.rs

@@ -0,0 +1,57 @@
+extern crate futures;
+extern crate tokio;
+extern crate tokio_executor;
+extern crate tokio_timer;
+
+#[macro_use]
+mod support;
+use support::*;
+
+use futures::{
+    prelude::*,
+    sync::mpsc,
+};
+use tokio::util::StreamExt;
+
+#[test]
+fn throttle() {
+    mocked(|timer, _| {
+        let (tx, rx) = mpsc::unbounded();
+        let mut stream = rx.throttle(ms(1))
+            .map_err(|e| panic!("{:?}", e));
+
+        assert_not_ready!(stream);
+
+        for i in 0..3 {
+            tx.unbounded_send(i).unwrap();
+        }
+        for i in 0..3 {
+            assert_ready_eq!(stream, Some(i));
+            assert_not_ready!(stream);
+
+            advance(timer, ms(1));
+        }
+
+        assert_not_ready!(stream);
+    });
+}
+
+#[test]
+fn throttle_dur_0() {
+    mocked(|_, _| {
+        let (tx, rx) = mpsc::unbounded();
+        let mut stream = rx.throttle(ms(0))
+            .map_err(|e| panic!("{:?}", e));
+
+        assert_not_ready!(stream);
+
+        for i in 0..3 {
+            tx.unbounded_send(i).unwrap();
+        }
+        for i in 0..3 {
+            assert_ready_eq!(stream, Some(i));
+        }
+
+        assert_not_ready!(stream);
+    });
+}