Create a "relation mapping" infrastructure to support changing the relfilenodes

of shared or nailed system catalogs.  This has two key benefits:

* The new CLUSTER-based VACUUM FULL can be applied safely to all catalogs.

* We no longer have to use an unsafe reindex-in-place approach for reindexing
  shared catalogs.

CLUSTER on nailed catalogs now works too, although I left it disabled on
shared catalogs because the resulting pg_index.indisclustered update would
only be visible in one database.

Since reindexing shared system catalogs is now fully transactional and
crash-safe, the former special cases in REINDEX behavior have been removed;
shared catalogs are treated the same as non-shared.

This commit does not do anything about the recently-discussed problem of
deadlocks between VACUUM FULL/CLUSTER on a system catalog and other
concurrent queries; will address that in a separate patch.  As a stopgap,
parallel_schedule has been tweaked to run vacuum.sql by itself, to avoid
such failures during the regression tests.

src/include/utils/relmapper.h

@@ -0,0 +1,62 @@
+/*-------------------------------------------------------------------------
+ *
+ * relmapper.h
+ *	  Catalog-to-filenode mapping
+ *
+ *
+ * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/utils/relmapper.h,v 1.1 2010/02/07 20:48:13 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef RELMAPPER_H
+#define RELMAPPER_H
+
+#include "access/xlog.h"
+
+/* ----------------
+ *		relmap-related XLOG entries
+ * ----------------
+ */
+
+#define XLOG_RELMAP_UPDATE		0x00
+
+typedef struct xl_relmap_update
+{
+	Oid			dbid;			/* database ID, or 0 for shared map */
+	Oid			tsid;			/* database's tablespace, or pg_global */
+	int32		nbytes;			/* size of relmap data */
+	char		data[1];		/* VARIABLE LENGTH ARRAY */
+} xl_relmap_update;
+
+#define MinSizeOfRelmapUpdate offsetof(xl_relmap_update, data)
+
+
+extern Oid	RelationMapOidToFilenode(Oid relationId, bool shared);
+
+extern void RelationMapUpdateMap(Oid relationId, Oid fileNode, bool shared,
+								 bool immediate);
+
+extern void RelationMapRemoveMapping(Oid relationId);
+
+extern void RelationMapInvalidate(bool shared);
+extern void RelationMapInvalidateAll(void);
+
+extern void AtCCI_RelationMap(void);
+extern void AtEOXact_RelationMap(bool isCommit);
+extern void AtPrepare_RelationMap(void);
+
+extern void CheckPointRelationMap(void);
+
+extern void RelationMapFinishBootstrap(void);
+
+extern void RelationMapInitialize(void);
+extern void RelationMapInitializePhase2(void);
+extern void RelationMapInitializePhase3(void);
+
+extern void relmap_redo(XLogRecPtr lsn, XLogRecord *record);
+extern void relmap_desc(StringInfo buf, uint8 xl_info, char *rec);
+
+#endif   /* RELMAPPER_H */