A BatchedDeleteStage can return PlanStage::IS_EOF in two different scenarios
(1) the child returned IS_EOF, there is nothing left to fetch, and the buffer has been drained
(2) a pass target was met, and even if there are more documents to fetch from the child, the pass is done 

For batched deletes that specify pass targets, such as the TTLMonitor, it is useful to differentiate between the two scenarios to know if the command should be executed again