(stable) 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 everything.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13152

Differential Revision: https://secure.phabricator.com/D19479