Fix the bug when aggregation stops before a group is finished

[pengpeng-lu]

• Problem: in aggregate queries, if a scan is stopped in the middle of a group, we'll just return the current result, which is wrong, and continuation = current row, so the next scan will also return wrong result.
• Solution: doesn't return anything if we stop in the middle of a group, and resets the continuation to the last group, to prepare for the next scan.

This resolves #3180

[alecgrieser]

It looks like there's a conflict in the release notes. PRs no longer need to update the release notes file as they will be auto-generated by #3174, so I think the solution is just "take what is on main"