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/mod.rs

@@ -41,9 +41,9 @@ mod stack;
 use self::atomic_stack::AtomicStack;
 use self::entry::Entry;
 use self::stack::Stack;
-use self::handle::HandlePriv;
 
 pub use self::handle::{Handle, with_default};
+pub(crate) use self::handle::HandlePriv;
 pub use self::now::{Now, SystemNow};
 pub(crate) use self::registration::Registration;