* Fix races.
This mostly pulls in changes from rust-lang-nursery/futures-rs#881, but
also updates Registration to be a bit more obvious as to what is going
on.
* Reduce spurious wakeups caused by Reactor
This patch adds an ABA guard on token values before registering them
with Mio. This allows catching token reuse and avoid the notification.
This is needed for OS X as the notification is used to determine that a
TCP connect has completed. A spurious notification can potentially cause
write failures.
This allows libraries that require access to reactor related types to
depend on this crate without having to depend on the entirety of Tokio.
For example, libraries that implement their custom I/O resource will
need to access `Registration` or `PollEvented`.
Carl Lerche