Get rid of pg_class.reltoastidxid.

Treat TOAST index just the same as normal one and get the OID
of TOAST index from pg_index but not pg_class.reltoastidxid.
This change allows us to handle multiple TOAST indexes, and
which is required infrastructure for upcoming
REINDEX CONCURRENTLY feature.

Patch by Michael Paquier, reviewed by Andres Freund and me.

src/include/access/tuptoaster.h

@@ -15,6 +15,7 @@
 
 #include "access/htup_details.h"
 #include "utils/relcache.h"
+#include "storage/lock.h"
 
 /*
  * This enables de-toasting of index entries.  Needed until VACUUM is
@@ -193,4 +194,12 @@ extern Size toast_raw_datum_size(Datum value);
  */
 extern Size toast_datum_size(Datum value);
 
+/* ----------
+ * toast_get_valid_index -
+ *
+ *	Return OID of valid index associated to a toast relation
+ * ----------
+ */
+extern Oid toast_get_valid_index(Oid toastoid, LOCKMODE lock);
+
 #endif   /* TUPTOASTER_H */