Change seqscan logic so that we check visibility of all tuples on a page

when we first read the page, rather than checking them one at a time.
This allows us to take and release the buffer content lock just once
per page, instead of once per tuple.  Since it's a shared lock the
contention penalty for holding the lock longer shouldn't be too bad.
We can safely do this only when using an MVCC snapshot; else the
assumption that visibility won't change over time is uncool.  Therefore
there are now two code paths depending on the snapshot type.  I also
made the same change in nodeBitmapHeapscan.c, where it can be done always
because we only support MVCC snapshots for bitmap scans anyway.
Also make some incidental cleanups in the APIs of these functions.
Per a suggestion from Qingqing Zhou.

src/include/access/valid.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/valid.h,v 1.36 2004/12/31 22:03:21 pgsql Exp $
+ * $PostgreSQL: pgsql/src/include/access/valid.h,v 1.37 2005/11/26 03:03:07 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -65,37 +65,4 @@ do \
 	} \
 } while (0)
 
-/*
- *		HeapTupleSatisfies
- *
- *	res is set TRUE if the HeapTuple satisfies the timequal and keytest,
- *	otherwise it is set FALSE.	Note that the hint bits in the HeapTuple's
- *	t_infomask may be updated as a side effect.
- *
- *	on 8/21/92 mao says:  i rearranged the tests here to do keytest before
- *	SatisfiesTimeQual.	profiling indicated that even for vacuumed relations,
- *	time qual checking was more expensive than key testing.  time qual is
- *	least likely to fail, too.	we should really add the time qual test to
- *	the restriction and optimize it in the normal way.	this has interactions
- *	with joey's expensive function work.
- */
-#define HeapTupleSatisfies(tuple, \
-						   relation, \
-						   buffer, \
-						   disk_page, \
-						   snapshot, \
-						   nKeys, \
-						   key, \
-						   res) \
-do \
-{ \
-	if ((key) != NULL) \
-		HeapKeyTest(tuple, RelationGetDescr(relation), nKeys, key, res); \
-	else \
-		(res) = true; \
- \
-	if ((res) && (relation)->rd_rel->relkind != RELKIND_UNCATALOGED) \
-		(res) = HeapTupleSatisfiesVisibility(tuple, snapshot, buffer); \
-} while (0)
-
 #endif   /* VALID_H */