Create a syscache for pg_database-indexed-by-oid, and make use of it

in various places that were previously doing ad hoc pg_database searches.
This may speed up database-related privilege checks a little bit, but
the main motivation is to eliminate the performance reason for having
ReverifyMyDatabase do such a lot of stuff (viz, avoiding repeat scans
of pg_database during backend startup).  The locking reason for having
that routine is about to go away, and it'd be good to have the option
to break it up.

src/backend/utils/cache/syscache.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/syscache.c,v 1.102 2006/03/05 15:58:45 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/syscache.c,v 1.103 2006/05/03 22:45:26 tgl Exp $
  *
  * NOTES
  *	  These routines allow the parser/planner/executor to perform
@@ -31,6 +31,7 @@
 #include "catalog/pg_auth_members.h"
 #include "catalog/pg_cast.h"
 #include "catalog/pg_conversion.h"
+#include "catalog/pg_database.h"
 #include "catalog/pg_index.h"
 #include "catalog/pg_inherits.h"
 #include "catalog/pg_language.h"
@@ -273,6 +274,16 @@ static const struct cachedesc cacheinfo[] = {
 			0,
 			0
 	}},
+	{DatabaseRelationId,		/* DATABASEOID */
+		DatabaseOidIndexId,
+		0,
+		1,
+		{
+			ObjectIdAttributeNumber,
+			0,
+			0,
+			0
+	}},
 	{IndexRelationId,			/* INDEXRELID */
 		IndexRelidIndexId,
 		Anum_pg_index_indrelid,