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/commands/vacuum.h

@@ -149,6 +149,7 @@ extern double vac_estimate_reltuples(Relation relation, bool is_analyze,
 extern void vac_update_relstats(Relation relation,
 					BlockNumber num_pages,
 					double num_tuples,
+					BlockNumber num_all_visible_pages,
 					bool hasindex,
 					TransactionId frozenxid);
 extern void vacuum_set_xid_limits(int freeze_min_age, int freeze_table_age,