Rewrite the sinval messaging mechanism to reduce contention and avoid

unnecessary cache resets.  The major changes are:

* When the queue overflows, we only issue a cache reset to the specific
backend or backends that still haven't read the oldest message, rather
than resetting everyone as in the original coding.

* When we observe backend(s) falling well behind, we signal SIGUSR1
to only one backend, the one that is furthest behind and doesn't already
have a signal outstanding for it.  When it finishes catching up, it will
in turn signal SIGUSR1 to the next-furthest-back guy, if there is one that
is far enough behind to justify a signal.  The PMSIGNAL_WAKEN_CHILDREN
mechanism is removed.

* We don't attempt to clean out dead messages after every message-receipt
operation; rather, we do it on the insertion side, and only when the queue
fullness passes certain thresholds.

* Split SInvalLock into SInvalReadLock and SInvalWriteLock so that readers
don't block writers nor vice versa (except during the infrequent queue
cleanout operations).

* Transfer multiple sinval messages for each acquisition of a read or
write lock.

src/include/storage/sinval.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/sinval.h,v 1.47 2008/03/16 19:47:34 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/storage/sinval.h,v 1.48 2008/06/19 21:32:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -83,7 +83,8 @@ typedef union
 } SharedInvalidationMessage;
 
 
-extern void SendSharedInvalidMessage(SharedInvalidationMessage *msg);
+extern void SendSharedInvalidMessages(const SharedInvalidationMessage *msgs,
+									  int n);
 extern void ReceiveSharedInvalidMessages(
 					  void (*invalFunction) (SharedInvalidationMessage *msg),
 							 void (*resetFunction) (void));