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
for root entity it retrieves single row, transforms it,
and loop until transfomer passes the first row
for child entities whole page is pulled. where the page is non-null children entity rows.
then the whole page is transformed and emitted to the row cache.
the rationale is avoid stealing child rows by parent entity threads. For every parent row
the linked children rows (page) is pulled under lock obtained on delegate EntityProcessor
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