Refactoring query_planner() callback

While hacking around the planner, the "callback" mechanism in 
query_planner started to feel awkward. It would seem more natural to 
split query_planner() into two parts: one function to do all the 
pre-processing of the jointree, building equivalence classes etc. And a 
second function to generate the Paths.

So where the callers currently do this:

     /* set up callback arguments */
     qp_extra.foo = ...;
     qp_extra.bar = ...;

     query_planner(root, qp_callback, &qp_extra);

static void
qp_callback(PlannerInfo *root, void *qp_extra)
{
   root->sort_pathkeys = ...;
   root->query_pathkeys = ...;
}


This would feel more natural to me:

     /* process the jointree, build equivalence classes */
     process_jointree(root);

     /* set up query_pathkeys */
     root->sort_pathkeys = ...;
     root->query_pathkeys = ...;

     query_planner(root);


Attached is a more full-fledged patch to do that.

The callback was introduced by commit db9f0e1d9a. The commit message 
explains the choice to use a callback like this:

> There are several ways in which we could implement that without making
> query_planner itself deal with grouping/sorting features (which are
> supposed to be the province of grouping_planner).  I chose to add a
> callback function to query_planner's API; other alternatives would have
> required adding more fields to PlannerInfo, which while not bad in itself
> would create an ABI break for planner-related plugins in the 9.2 release
> series.  This still breaks ABI for anything that calls query_planner
> directly, but it seems somewhat unlikely that there are any such plugins.

In v12, we can change the ABI.

- Heikki