final T prepareObjectForNotification(T o, boolean withPooledObjects) {
        onRemoval(o);
        return touchOrCopyAndClose(o, withPooledObjects);
    }
Invoked after an element is removed from the {@link StreamMessage} and before
{@link Subscriber#onNext(Object)} is invoked.

@param obj the removed element
core/src/main/java/com/linecorp/armeria/common/stream/CancellableStreamMessage.java