Moving SS_finalize_plan processing to the end of planning

I've started to work on path-ification of the upper planner (finally),
and since that's going to be a large patch in any case, I've been looking
for pieces that could be bitten off and done separately. One such piece
is the fact that SS_finalize_plan (computation of extParams/allParams)
currently gets run at the end of subquery_planner; as long as that's true,
we cannot have subquery_planner returning paths rather than concrete
plans. The attached patch moves that processing to occur just before
set_plan_references is run.

Ideally I'd like to get rid of SS_finalize_plan altogether and merge its
work into set_plan_references, so as to save one traversal of the finished
plan tree. However, that's a bit difficult because of the fact that the
traversal order is different: in SS_finalize_plan, we must visit subplans
before main plan, whereas set_plan_references wants to do the main plan
first. (I experimented with changing that, but then the flat rangetable
comes out in a different order, with unpleasant side effects on the way
EXPLAIN prints things.) Since that would be purely a minor performance
improvement, I set that goal aside for now.

Basically what this patch does is to divide what had been done in
SS_finalize_plan into three steps:

* SS_identify_outer_params determines which outer-query-level Params will
be available for the current query level to use, and saves that aside in
a new PlannerInfo field root->outer_params. This processing turns out
to be the only reason that SS_finalize_plan had to be called in
subquery_planner: we have to capture this data before exiting
subquery_planner because the upper levels' plan_params lists may change
as they plan other subplans.

* SS_attach_initplans does the work of attaching initplans to the parent
plan node and adjusting the parent's cost estimate accordingly.

* SS_finalize_plan now *only* does extParam/allParam calculation.

I had to change things around a bit in planagg.c (which was already a
hack, and the law of conservation of cruft applies). Otherwise it's
pretty straightforward and removes some existing hacks. One notable
point is that there's no longer an assumption within SS_finalize_plan
that initPlans can only appear in the top plan node.

Any objections?

regards, tom lane