private boolean subscribe0(DownstreamSubscription<T> subscription) {
            if (!subscriptionUpdater.compareAndSet(this, null, subscription)) {
                return false;
            }

            processor.subscribe(subscription);
            return true;
        }
Returns the default {@link EventExecutor} which will be used when a user subscribes to a child
stream using {@link StreamMessage#subscribe(Subscriber, SubscriptionOption...)}.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java