Get rid of pg_class.reltoastidxid.

 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.