void unsubscribe(@Nullable Throwable cause) {
            if (downstreamExecutor.inEventLoop()) {
                doUnsubscribe(cause);
            } else {
                downstreamExecutor.execute(() -> doUnsubscribe(cause));
            }
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java