time: do not panic on timeout(Duration::MAX) (#3551)

It is tempting to use very large `Duration` value to get a practically
infinite timeout.

Before this commit Tokio panics on checked Instant + Duration
overflow.

This commit implements very simple fix: if Instant + Duration
overflows, we use duration = 30 years. Better fix should avoid
firing a timer on duration overflow. It requires deeper understanding
how timers work, but also it is not clear, for example, what
`Sleep::deadline` function should return.

Similar fix is done for `sleep`.

tokio/tests/time_timeout.rs

@@ -74,6 +74,33 @@ async fn future_and_timeout_in_future() {
     assert_ready_ok!(fut.poll()).unwrap();
 }
 
+#[tokio::test]
+async fn very_large_timeout() {
+    time::pause();
+
+    // Not yet complete
+    let (tx, rx) = oneshot::channel();
+
+    // copy-paste unstable `Duration::MAX`
+    let duration_max = Duration::from_secs(u64::MAX) + Duration::from_nanos(999_999_999);
+
+    // Wrap it with a deadline
+    let mut fut = task::spawn(timeout(duration_max, rx));
+
+    // Ready!
+    assert_pending!(fut.poll());
+
+    // Turn the timer, it runs for the elapsed time
+    time::advance(Duration::from_secs(86400 * 365 * 10)).await;
+
+    assert_pending!(fut.poll());
+
+    // Complete the future
+    tx.send(()).unwrap();
+
+    assert_ready_ok!(fut.poll()).unwrap();
+}
+
 #[tokio::test]
 async fn deadline_now_elapses() {
     use futures::future::pending;