Re: Eager aggregation, take 3

Here is an update of the patchset with the following changes:

* Fix a 'Aggref found where not expected' error caused by the PVC call
in is_var_in_aggref_only.  This would happen if we have Aggrefs
contained in other expressions.

* Use joinrel's relids rather than the union of the relids of its outer
and inner to search for its grouped rel.  This is more correct as we
need to include OJs into consideration.

* Remove RelAggInfo.agg_exprs as it is not used anymore.

Thanks
Richard