sql: Rework top N window functions.

[jacksonrnewhouse]

This allows users to sort window functions in the SlidingTopN case, so queries like

SELECT * FROM (
SELECT ROW_NUMBER() OVER (
    PARTITION BY window
    ORDER BY count DESC, auction) as row_num, *
FROM (SELECT count(*) as count,
    bid.auction as auction,
    hop(interval '5 seconds', interval '15 seconds') as window
        FROM nexmark
        group by 2,3)) WHERE row_num < 2

I also added a clone to some of the partial aggregate code, so that you can aggregate strings, like bid.extra.