Support index-only scans using the visibility map to avoid heap fetches.

When a btree index contains all columns required by the query, and the
visibility map shows that all tuples on a target heap page are
visible-to-all, we don't need to fetch that heap page.  This patch depends
on the previous patches that made the visibility map reliable.

There's a fair amount left to do here, notably trying to figure out a less
chintzy way of estimating the cost of an index-only scan, but the core
functionality seems ready to commit.

Robert Haas and Ibrar Ahmed, with some previous work by Heikki Linnakangas.

src/backend/access/index/genam.c

@@ -93,6 +93,8 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 	else
 		scan->orderByData = NULL;
 
+	scan->xs_want_itup = false;			/* may be set later */
+
 	/*
 	 * During recovery we ignore killed tuples and don't bother to kill them
 	 * either. We do this because the xmin on the primary node could easily be
@@ -109,6 +111,8 @@ RelationGetIndexScan(Relation indexRelation, int nkeys, int norderbys)
 
 	scan->opaque = NULL;
 
+	scan->xs_itup = NULL;
+
 	ItemPointerSetInvalid(&scan->xs_ctup.t_self);
 	scan->xs_ctup.t_data = NULL;
 	scan->xs_cbuf = InvalidBuffer;