Implement new 'lightweight lock manager' that's intermediate between

existing lock manager and spinlocks: it understands exclusive vs shared
lock but has few other fancy features.  Replace most uses of spinlocks
with lightweight locks.  All remaining uses of spinlocks have very short
lock hold times (a few dozen instructions), so tweak spinlock backoff
code to work efficiently given this assumption.  All per my proposal on
pghackers 26-Sep-01.

src/include/access/transam.h

@@ -7,15 +7,13 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: transam.h,v 1.40 2001/08/26 16:56:00 tgl Exp $
+ * $Id: transam.h,v 1.41 2001/09/29 04:02:26 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 #ifndef TRANSAM_H
 #define TRANSAM_H
 
-#include "storage/spin.h"
-
 
 /* ----------------
  *		Special transaction ID values
@@ -122,8 +120,6 @@ extern void CheckMaxObjectId(Oid assigned_oid);
 extern bool AMI_OVERRIDE;
 
 /* in varsup.c */
-extern SPINLOCK OidGenLockId;
-extern SPINLOCK XidGenLockId;
 extern VariableCache ShmemVariableCache;
 
 #endif	 /* TRAMSAM_H */