timer: Reduce size of Delay struct (#554)

* Remove `counted` field on `timer::Entry`.

It turns out that a better indicator of whether or not the number of
active timeouts should be decremented is if the `Entry` has been
associated with a timer. In other words, if `Entry::inner` can be
upgraded, then the count should be decremented on drop.

* timer: Tweak link between `Delay` and the driver

This tweaks the struct layout / details regarding how a `Delay` instance
is linked to a driver (timer instance). Instead of lazily allocating the
`Entry` (node shared between `Delay` and the timer), `Entry` is
allocated immediately when `Delay` is created. This allows using the
entry store data used by `Delay`.

This is in anticipation of further timer improvements that would
otherwise require the size of `Delay` to grow further. Since an
allocation is already made, the idea is to shrink the size of the
`Delay` struct.

tokio-timer/src/timer/handle.rs

@@ -1,5 +1,5 @@
 use {Error, Delay, Deadline, Interval};
-use timer::{Registration, Inner};
+use timer::Inner;
 
 use tokio_executor::Enter;
 
@@ -128,11 +128,7 @@ impl Handle {
     pub fn delay(&self, deadline: Instant) -> Delay {
         match self.inner {
             Some(ref handle_priv) => {
-                let registration = Registration::new_with_handle(
-                    deadline,
-                    handle_priv.clone());
-
-                Delay::new_with_registration(deadline, registration)
+                Delay::new_with_handle(deadline, handle_priv.clone())
             }
             None => {
                 Delay::new(deadline)