Support pushing down date_trunc(<unit>, ts) dyn filter to the downstream operators

[niebayes]

For a query that aggregates by date_trunc(<unit>, ts) and takes the top-K groups by the same expression, e.g.

SELECT date_trunc('minute', ts) AS minute, max(usage_user)
FROM cpu
WHERE ts < '2016-01-01T14:41:34Z'
GROUP BY minute
ORDER BY minute DESC
LIMIT 5;

the SortExec (TopK fetch=5) builds a DynamicFilterPhysicalExpr on minute. Even though minute is exactly the aggregate's grouping expression, this dynamic filter is not pushed down past the AggregateExec to the scan. As a result the scan reads the entire input instead of only the newest minute buckets. This is the dominant cost for TSBS benchmark case groupby-orderby-limit.

Expected behavior

The TopK filter on the grouping expression should be pushed below the aggregate (and below the projection), because it only selects which groups to compute — each group's aggregate value is identical whether the filter is applied before or after grouping. Ideally the filter should also be rewritten so a scan can evaluate it, i.e. the predicate on

date_trunc(k, ts) <cmp> X

should be exposed as an equivalent predicate on the base column ts, using monotonicity of date_trunc:

filter on date_trunc(k, ts)	equivalent on ts
> X	ts >= X + k
>= X	ts >= X
< X	ts < X
<= X	ts < X + k

This is exactly the "top-K by time bucket" pattern, where only the newest N buckets need to be scanned.