Re: Add proper planner support for ORDER BY / DISTINCT aggregates

I've been working on this patch again. There was a bit of work to do
to rebase it atop db0d67db2.  The problem there was that since this
patch appends pathkeys to suit ORDER BY / DISTINCT aggregates to the
query's group_pathkeys, db0d67db2 goes and tries to rearrange those,
but fails to find the SortGroupClause corresponding to the PathKey in
group_pathkeys. I wish the code I came up with to make that work was a
bit nicer, but what's there at least seems to work. There are a few
more making copies of Lists than I'd like.

We may need to do more checks when adding members to 'aggindexes' to
record we've found pathkeys for an aggregate, because 'currpathkeys' may
include pathkeys for some latter aggregates. I can see this problem with
the query below:

    select max(b order by b), max(a order by a) from t group by a;

When processing the first aggregate, we compose the 'currpathkeys' as
{a, b} and mark this aggregate in 'aggindexes'. When it comes to the
second aggregate, we compose its pathkeys as {a} and decide that it is
not stronger than 'currpathkeys'. So the second aggregate is not
recorded in 'aggindexes'. As a result, we fail to mark aggpresorted for
the second aggregate.

Thanks