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/utility.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/utility.h,v 1.31 2007/02/20 17:32:17 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/tcop/utility.h,v 1.32 2007/03/13 00:33:43 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -17,8 +17,9 @@
 #include "tcop/tcopprot.h"
 
 
-extern void ProcessUtility(Node *parsetree, ParamListInfo params,
-			   DestReceiver *dest, char *completionTag);
+extern void ProcessUtility(Node *parsetree, const char *queryString,
+						   ParamListInfo params, bool isTopLevel,
+						   DestReceiver *dest, char *completionTag);
 
 extern bool UtilityReturnsTuples(Node *parsetree);