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?
Do you have to use mocks here? what about asserting that the cache entries existed and there were actually deleted?