private void notifyAwaitDemandFuture() {
        @SuppressWarnings("unchecked")
        final CompletableFuture<Void> f = (CompletableFuture<Void>) queue.remove();
        f.complete(null);
    }
Invoked whenever a new demand is requested.
@param n Newly requested demand
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessage.java