Port to Drupal 8

This module has the right approach, and we still need it in Drupal 8.

So, let's use this issue to track a Drupal 8 port. We can then try to get it in core for 8.1.

This time around the module can be entity reference specific.
The tricky part is porting field_referenced_delete_get_references() since EntityQuery no longer supports querying across entity types. We'll need to get all entityreference fields targeting our entity type. This makes me worried about performance as well, since we'll end up with an EntityQuery per entity type.