R1:0a3093ef9c18

Fix an issue where paginating notifications could fail a GROUP BY test

Summary:
Ref T13623. When paginating notifications, we may currently construct a query which:

- loads from non-unique rows; and
- returns multiple results.

In particular, `chronologicalKey` isn't unique across the whole table (only for a given viewer). We can get away with this because no user-facing view of notifications is truly "every notification for every viewer" today.

One fix would be to implicitly force the paging query…