void clearSubscriber() {
            if (!(subscriber instanceof AbortingSubscriber)) {
                subscriber = NeverInvokedSubscriber.get();
            }
        }
Replaces the subscriber with a placeholder so that it can be garbage-collected and
we conform to the Reactive Streams specification rule 3.13.
core/src/main/java/com/linecorp/armeria/common/stream/CancellableStreamMessage.java