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/storage/smgr.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: smgr.h,v 1.30 2001/06/27 23:31:39 tgl Exp $
+ * $Id: smgr.h,v 1.31 2001/09/29 04:02:27 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -17,9 +17,9 @@
 #include "access/xlog.h"
 #include "storage/relfilenode.h"
 #include "storage/block.h"
-#include "storage/spin.h"
 #include "utils/rel.h"
 
+
 #define SM_FAIL			0
 #define SM_SUCCESS		1
 
@@ -79,8 +79,6 @@ extern int	mdabort(void);
 extern int	mdsync(void);
 
 /* mm.c */
-extern SPINLOCK MMCacheLock;
-
 extern int	mminit(void);
 extern int	mmcreate(Relation reln);
 extern int	mmunlink(RelFileNode rnode);