private void pushSignal(Object obj) {
            if (executor.inEventLoop()) {
                doPushSignal(obj);
            } else {
                executor.execute(() -> doPushSignal(obj));
            }
        }
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