further fixes to the async Close method

async.go

@@ -47,17 +47,10 @@ func NewAsyncTopic[T any](opts ...TopicOption) *AsyncTopic[T] {
 // Close terminates background go routines and prevents further publishing and subscribing. All
 // published messages are garanteed to be delivered once Close returns. This is idempotent.
 func (t *AsyncTopic[T]) Close() {
-	t.mu.RLock()
-	closing := t.closing
-	t.mu.RUnlock()
-	if closing {
-		// It's either already closed or it's closing.
-		return
-	}
-
 	t.mu.Lock()
-	if closing {
+	if t.closing {
-		// It is possible that 2 go routines attempted to aquire the lock to close this topic.
+		// Multiple go routines attempted to close this topic. Both should wait for the topic to be
+		// closed before returning.
 		t.mu.Unlock()
 		<-t.closed
 		return