Re: Eager aggregation, take 3

Another rebase is needed after d1d286d83c.  Also I realized that the
partially_grouped_rel generated by eager aggregation might be dummy,
such as in query:

select count(t2.c) from t t1 join t t2 on t1.b = t2.b where false group by t1.a;

If somehow we choose this dummy path with a Finalize Agg Path on top of
it as the final cheapest path (a very rare case), we would encounter the
"Aggref found in non-Agg plan node" error.  The v7 patch fixes this
issue.

Thanks
Richard