Inline agg

This is part 1: Serial Inline aggregation contain table agg and group by agg with index.

Waiting for Jenkins.

Also, I don't see any logic here that prevents pushdown of a limit in the case that we have a GROUP BY but no ORDER BY. If I have no ORDER BY and no DISTINCT agg and I have a GROUP BY that does not contain the partition key, it seems like I can only push down a limit if I also push down an artificially  forced ORDER BY on the group keys -- and I don't see that kind of code anywhere. Otherwise, partitions 1 and 2 might have the same 12 group keys, but with LIMIT 10, partition 1 might send  groups 1 - 10 and not 11-12 but partition 2 only sends me 3-12 but not 1-2, so the coordinator can not reliably produce 10 complete groups. Another existing bug? Or am I missing something?