Generalize concept of temporary relations to "relation persistence".

This commit replaces pg_class.relistemp with pg_class.relpersistence;
and also modifies the RangeVar node type to carry relpersistence rather
than istemp.  It also removes removes rd_istemp from RelationData and
instead performs the correct computation based on relpersistence.

For clarity, we add three new macros: RelationNeedsWAL(),
RelationUsesLocalBuffers(), and RelationUsesTempNamespace(), so that we
can clarify the purpose of each check that previous depended on
rd_istemp.

This is intended as infrastructure for the upcoming unlogged tables
patch, as well as for future possible work on global temporary tables.

src/include/nodes/primnodes.h

@@ -74,7 +74,7 @@ typedef struct RangeVar
 	char	   *relname;		/* the relation/sequence name */
 	InhOption	inhOpt;			/* expand rel by inheritance? recursively act
 								 * on children? */
-	bool		istemp;			/* is this a temp relation/sequence? */
+	char		relpersistence;	/* see RELPERSISTENCE_* in pg_class.h */
 	Alias	   *alias;			/* table alias & optional column aliases */
 	int			location;		/* token location, or -1 if unknown */
 } RangeVar;