Changing task::block_on to park after a single poll

This was previously discussed in #605 and others as a source of high
CPU load when sleeping tasks because of the overhead created by
retrying a future in short succession.

Author: spacekookie

Diff for: src/task/block_on.rs

@@ -125,22 +125,14 @@ where
         let waker = unsafe { ManuallyDrop::new(Waker::from_raw(RawWaker::new(ptr, &VTABLE))) };
         let cx = &mut Context::from_waker(&waker);
 
-        let mut step = 0;
         loop {
             if let Poll::Ready(t) = future.as_mut().poll(cx) {
                 // Save the parker for the next invocation of `block`.
                 cache.set(Some(arc_parker));
                 return t;
             }
 
-            // Yield a few times or park the current thread.
-            if step < 3 {
-                thread::yield_now();
-                step += 1;
-            } else {
-                arc_parker.park();
-                step = 0;
-            }
+            arc_parker.park();
         }
     })
 }