When a batch from an orphaned range is cleared up, the numOrphanDocs field is updated on the corresponding range deletion task document.

During upgrade such field is populated by counting orphaned docs via an index scan and during downgrade such field is unset.

In case a range deletion task document being updated as part of upgrade/downgrade is also undergoing a batch deletion at the same time, the the update operations may overlap by leaving the orphaned docs count in an inconsistent state.