Re: Eager aggregation, take 3

This needs another rebase after 97d85be365.  I also addressed several
issues that I identified during self-review, which include:

* In some cases GroupPathExtraData.agg_final_costs, which is the cost of
final aggregation, fails to be calculated.  This can lead to bogus cost
estimation and end up with unexpected plan.

* If the cheapest partially grouped path is generated through eager
aggregation, the number of groups estimated for the final phase will be
different from the number of groups estimated for non-split aggregation.
That is to say, we should not use 'dNumGroups' for the final aggregation
in add_paths_to_grouping_rel().

* It is possible that we may generate dummy grouped join relations, and
that would trigger the Assert in make_grouped_join_rel().

* More typo fixes.

Thanks
Richard