void doAbort(Throwable cause) {
            if (state != State.ABORTED) {
                state = State.ABORTED;
                abortCause = cause;
                doCancelUpstreamSubscription();
                // Do not call 'upstream.abort();', but 'upstream.cancel()' because this is not aborting
                // the upstream StreamMessage, but aborting duplicator.
                doCleanup(cause);
            }
        }
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