First phase of plan-invalidation project: create a plan cache management

module and teach PREPARE and protocol-level prepared statements to use it.
In service of this, rearrange utility-statement processing so that parse
analysis does not assume table schemas can't change before execution for
utility statements (necessary because we don't attempt to re-acquire locks
for utility statements when reusing a stored plan).  This requires some
refactoring of the ProcessUtility API, but it ends up cleaner anyway,
for instance we can get rid of the QueryContext global.

Still to do: fix up SPI and related code to use the plan cache; I'm tempted to
try to make SQL functions use it too.  Also, there are at least some aspects
of system state that we want to ensure remain the same during a replan as in
the original processing; search_path certainly ought to behave that way for
instance, and perhaps there are others.

src/include/tcop/pquery.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/tcop/pquery.h,v 1.41 2007/02/20 17:32:17 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/tcop/pquery.h,v 1.42 2007/03/13 00:33:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -33,7 +33,7 @@ extern void PortalStart(Portal portal, ParamListInfo params,
 extern void PortalSetResultFormat(Portal portal, int nFormats,
 					  int16 *formats);
 
-extern bool PortalRun(Portal portal, long count,
+extern bool PortalRun(Portal portal, long count, bool isTopLevel,
 		  DestReceiver *dest, DestReceiver *altdest,
 		  char *completionTag);