time: clean time driver (#2905)

* remove unnecessary wheel::Poll

the timer wheel uses the `wheel::Poll` struct as input when
advancing the timer to the next time step.  the `Poll` struct
contains an instant representing the time step to advance to
and also contains an optional and mutable reference to an
`Expiration` struct.  from what I can tell, the latter field
is only used in the context of polling the wheel and does not
need to be exposed outside of that method.  without the
expiration field the `Poll` struct is nothing more than a
wrapper around the instant being polled.  this change removes
the `Poll` struct and updates integration points accordingly.

* remove Stack trait in favor of concrete Stack implementation

* remove timer Registration struct

tokio-util/src/time/delay_queue.rs

@@ -141,7 +141,7 @@ pub struct DelayQueue<T> {
     delay: Option<Delay>,
 
     /// Wheel polling state
-    poll: wheel::Poll,
+    wheel_now: u64,
 
     /// Instant at which the timer starts
     start: Instant,
@@ -251,7 +251,7 @@ impl<T> DelayQueue<T> {
             slab: Slab::with_capacity(capacity),
             expired: Stack::default(),
             delay: None,
-            poll: wheel::Poll::new(0),
+            wheel_now: 0,
             start: Instant::now(),
         }
     }
@@ -733,11 +733,11 @@ impl<T> DelayQueue<T> {
 
                 let now = crate::time::ms(delay.deadline() - self.start, crate::time::Round::Down);
 
-                self.poll = wheel::Poll::new(now);
+                self.wheel_now = now;
             }
 
             // We poll the wheel to get the next value out before finding the next deadline.
-            let wheel_idx = self.wheel.poll(&mut self.poll, &mut self.slab);
+            let wheel_idx = self.wheel.poll(self.wheel_now, &mut self.slab);
 
             self.delay = self.next_deadline().map(sleep_until);