void doClose() {
            if (state == State.DUPLICABLE) {
                if (duplicator.unsubscribed == 0 && downstreamSubscriptions.isEmpty()) {
                    state = State.ABORTED;
                    doCancelUpstreamSubscription();
                    signals.clear(null);
                } else {
                    state = State.CLOSED;
                }
            }
        }
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