Fix race condition related bugs (#243)

* 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.

tokio-threadpool/src/lib.rs

@@ -936,9 +936,10 @@ impl Future for Shutdown {
     type Error = ();
 
     fn poll(&mut self) -> Poll<(), ()> {
+        use futures::task;
         trace!("Shutdown::poll");
 
-        self.inner().shutdown_task.task1.register();
+        self.inner().shutdown_task.task1.register_task(task::current());
 
         if 0 != self.inner().num_workers.load(Acquire) {
             return Ok(Async::NotReady);

tokio-threadpool/src/task.rs

@@ -222,13 +222,13 @@ impl Task {
             let actual = self.inner().state.compare_and_swap(
                 Idle.into(),
                 Scheduled.into(),
-                Relaxed).into();
+                AcqRel).into();
 
             match actual {
                 Idle => return true,
                 Running => {
                     let actual = self.inner().state.compare_and_swap(
-                        Running.into(), Notified.into(), Relaxed).into();
+                        Running.into(), Notified.into(), AcqRel).into();
 
                     match actual {
                         Idle => continue,