R1:13dd3014a758

Fix loop in QueryIterator when row count is an exact multiple of page size

Summary: Ref T13152. The pager does a bit of magic here and doesn't populate `nextPageID` when it knows it got an exact final page. The logic misfired in this case and sent us back to the start.

Test Plan:
- Set page size to 1 to guarantee rows were an exact multiple of page size.
- Ran `rebuild-identities` (I no-op'd the actual logic to make it faster).
- Before: looped forever.
- After: clean exit after processing…