sync: add Notify::notify_waiters (#3098)

This PR makes `Notify::notify_waiters` public. The method
already exists, but it changes the way `notify_waiters`,
is used. Previously in order for the consumer to
register interest, in a notification triggered by
`notify_waiters`, the `Notified` future had to be
polled. This introduced friction when using the api
as the future had to be pinned before polled.

This change introduces a counter that tracks how many
times `notified_waiters` has been called. Upon creation of
the future the number of times is loaded. When first
polled the future compares this number with the count
state of the `Notify` type. This avoids the need for
registering the waiter upfront.

Fixes: #3066

tokio/src/sync/mpsc/chan.rs

@@ -148,25 +148,10 @@ impl<T, S: Semaphore> Tx<T, S> {
     }
 
     pub(crate) async fn closed(&self) {
-        use std::future::Future;
-        use std::pin::Pin;
-        use std::task::Poll;
-
         // In order to avoid a race condition, we first request a notification,
-        // **then** check the current value's version. If a new version exists,
-        // the notification request is dropped. Requesting the notification
-        // requires polling the future once.
+        // **then** check whether the semaphore is closed. If the semaphore is
+        // closed the notification request is dropped.
         let notified = self.inner.notify_rx_closed.notified();
-        pin!(notified);
-
-        // Polling the future once is guaranteed to return `Pending` as `watch`
-        // only notifies using `notify_waiters`.
-        crate::future::poll_fn(|cx| {
-            let res = Pin::new(&mut notified).poll(cx);
-            assert!(!res.is_ready());
-            Poll::Ready(())
-        })
-        .await;
 
         if self.inner.semaphore.is_closed() {
             return;