Restructure command destination handling so that we pass around

DestReceiver pointers instead of just CommandDest values.  The DestReceiver
is made at the point where the destination is selected, rather than
deep inside the executor.  This cleans up the original kluge implementation
of tstoreReceiver.c, and makes it easy to support retrieving results
from utility statements inside portals.  Thus, you can now do fun things
like Bind and Execute a FETCH or EXPLAIN command, and it'll all work
as expected (e.g., you can Describe the portal, or use Execute's count
parameter to suspend the output partway through).  Implementation involves
stuffing the utility command's output into a Tuplestore, which would be
kind of annoying for huge output sets, but should be quite acceptable
for typical uses of utility commands.

src/include/executor/execdesc.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2002, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: execdesc.h,v 1.22 2002/12/15 16:17:54 tgl Exp $
+ * $Id: execdesc.h,v 1.23 2003/05/06 20:26:28 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -33,7 +33,7 @@ typedef struct QueryDesc
 	CmdType		operation;		/* CMD_SELECT, CMD_UPDATE, etc. */
 	Query	   *parsetree;		/* rewritten parsetree */
 	Plan	   *plantree;		/* planner's output */
-	CommandDest dest;			/* the destination output of the execution */
+	DestReceiver *dest;			/* the destination for tuple output */
 	const char *portalName;		/* name of portal, or NULL */
 	ParamListInfo params;		/* param values being passed in */
 	bool		doInstrument;	/* TRUE requests runtime instrumentation */
@@ -46,7 +46,7 @@ typedef struct QueryDesc
 
 /* in pquery.c */
 extern QueryDesc *CreateQueryDesc(Query *parsetree, Plan *plantree,
-								  CommandDest dest, const char *portalName,
+								  DestReceiver *dest, const char *portalName,
 								  ParamListInfo params,
 								  bool doInstrument);