groupBy: push down limits when sorting on the group

[gianm]

@vogievetsky pointed out to me that it's possible to push down ordering + limits if you're sorting on the grouping tuple. This is because any tuple in the first N results overall is going to either be somewhere in the first N results from each segment, or not in that segment at all.

[gianm]

#2864 was about doing this on groupBy v1, but these days it makes more sense to work on groupBy v2. Doing limit pushdown there would involve implementing some kind of continuous sorting and limiting in the merge step, which could take the form of a priority-queue-powered version of BufferGrouper.

[gianm]

Fixed by #3873 (in groupBy v2).