sync: use intrusive list strategy for broadcast (#2509)

Previously, in the broadcast channel, receiver wakers were passed to the
sender via an atomic stack with allocated nodes. When a message was
sent, the stack was drained. This caused a problem when many receivers
pushed a waiter node then dropped. The waiter node remained indefinitely
in cases where no values were sent.

This patch switches broadcast to use the intrusive linked-list waiter
strategy used by `Notify` and `Semaphore.

tokio/src/loom/std/atomic_ptr.rs

@@ -11,10 +11,6 @@ impl<T> AtomicPtr<T> {
         let inner = std::sync::atomic::AtomicPtr::new(ptr);
         AtomicPtr { inner }
     }
-
-    pub(crate) fn with_mut<R>(&mut self, f: impl FnOnce(&mut *mut T) -> R) -> R {
-        f(self.inner.get_mut())
-    }
 }
 
 impl<T> Deref for AtomicPtr<T> {

tokio/src/sync/batch_semaphore.rs

@@ -186,7 +186,7 @@ impl Semaphore {
 
     /// Release `rem` permits to the semaphore's wait list, starting from the
     /// end of the queue.
-    ///  
+    ///
     /// If `rem` exceeds the number of permits needed by the wait list, the
     /// remainder are assigned back to the semaphore.
     fn add_permits_locked(&self, mut rem: usize, waiters: MutexGuard<'_, Waitlist>) {