Fix invalid parent cache invalidation

A relation embedded in 2 parents would only clear the cache in one of them.

Embedding twice an object would override `expire_parent_cache` and only the latest embedding parent would be cleared on child change.

This PR keeps these callback with unique names to prevent that clash.

@dylanahsmith What do you think?
I think you are right, must be a left-over from a previous implementation. I do have anything that comes to mind why we would need this.