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.

doc/src/sgml/catalogs.sgml

@@ -1654,6 +1654,19 @@
       </entry>
      </row>
 
+     <row>
+      <entry><structfield>relallvisible</structfield></entry>
+      <entry><type>int4</type></entry>
+      <entry></entry>
+      <entry>
+       Number of pages that are marked all-visible in the table's
+       visibility map.  This is only an estimate used by the
+       planner.  It is updated by <command>VACUUM</command>,
+       <command>ANALYZE</command>, and a few DDL commands such as
+       <command>CREATE INDEX</command>.
+      </entry>
+     </row>
+
      <row>
       <entry><structfield>reltoastrelid</structfield></entry>
       <entry><type>oid</type></entry>