Measure the number of all-visible pages for use in index-only scan costing.

Add a column pg_class.relallvisible to remember the number of pages that
were all-visible according to the visibility map as of the last VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to estimate
how many heap page fetches can be avoided during an index-only scan.

This is pretty primitive and will no doubt see refinements once we've
acquired more field experience with the index-only scan mechanism, but
it's way better than using a constant.

Note: I had to adjust an underspecified query in the window.sql regression
test, because it was changing answers when the plan changed to use an
index-only scan.  Some of the adjacent tests perhaps should be adjusted
as well, but I didn't do that here.

src/include/optimizer/plancat.h

@@ -29,7 +29,7 @@ extern void get_relation_info(PlannerInfo *root, Oid relationObjectId,
 				  bool inhparent, RelOptInfo *rel);
 
 extern void estimate_rel_size(Relation rel, int32 *attr_widths,
-				  BlockNumber *pages, double *tuples);
+				  BlockNumber *pages, double *tuples, double *allvisfrac);
 
 extern int32 get_relation_data_width(Oid relid, int32 *attr_widths);