timer: introduce delay function shortcut

This PR adds a simple delay shortcut to avoid writing Delay::new
everywhere.

Refs: #1261

tokio-timer/src/lib.rs

@@ -64,6 +64,11 @@ pub fn sleep(duration: Duration) -> Delay {
  Delay::new(Instant::now() + duration)
 }
 
+/// Create a Future that completes at `deadline`.
+pub fn delay(deadline: Instant) -> Delay {
+ Delay::new(deadline)
+}
+
 // ===== Internal utils =====
 
 enum Round {