Add a new upper planner relation for partially-aggregated results.

Up until now, we've abused grouped_rel->partial_pathlist as a place to
store partial paths that have been partially aggregate, but that's
really not correct, because a partial path for a relation is supposed
to be one which produces the correct results with the addition of only
a Gather or Gather Merge node, and these paths also require a Finalize
Aggregate step.  Instead, add a new partially_group_rel which can hold
either partial paths (which need to be gathered and then have
aggregation finalized) or non-partial paths (which only need to have
aggregation finalized).  This allows us to reuse generate_gather_paths
for partially_grouped_rel instead of writing new code, so that this
patch actually basically no net new code while making things cleaner,
simplifying things for pending patches for partition-wise aggregate.

Robert Haas and Jeevan Chalke.  The larger patch series of which this
patch is a part was also reviewed and tested by Antonin Houska,
Rajkumar Raghuwanshi, David Rowley, Dilip Kumar, Konstantin Knizhnik,
Pascal Legrand, Rafia Sabih, and me.

Discussion: http://postgr.es/m/CA+TgmobrzFYS3+U8a_BCy3-hOvh5UyJbC18rEcYehxhpw5=ETA@mail.gmail.com
Discussion: http://postgr.es/m/CA+TgmoZyQEjdBNuoG9-wC5GQ5GrO4544Myo13dVptvx+uLg9uQ@mail.gmail.com

src/include/optimizer/paths.h

@@ -53,7 +53,8 @@ extern void set_dummy_rel_pathlist(RelOptInfo *rel);
 extern RelOptInfo *standard_join_search(PlannerInfo *root, int levels_needed,
 					 List *initial_rels);
 
-extern void generate_gather_paths(PlannerInfo *root, RelOptInfo *rel);
+extern void generate_gather_paths(PlannerInfo *root, RelOptInfo *rel,
+					  bool override_rows);
 extern int compute_parallel_worker(RelOptInfo *rel, double heap_pages,
 						double index_pages, int max_workers);
 extern void create_partial_bitmap_paths(PlannerInfo *root, RelOptInfo *rel,