This method is called when it starts processing an entity. When it comes
back to the entity it is called again. So it can reset anything at that point.
For a rootmost entity this is called only once for an ingestion. For sub-entities , this
is called multiple once for each row from its parent entity
This is used during delta-import. It gives the primary keys of the rows
that are deleted from this entity. If this entity is the root entity, solr
document is deleted. If this is a sub-entity, the Solr document is
considered as 'changed' and will be recreated