Handle heap rewrites even better in logical decoding

Logical decoding should not publish anything about tables created as
part of a heap rewrite during DDL.  Those tables don't exist externally,
so consumers of logical decoding cannot do anything sensible with that
information.  In ab28feae2b, we worked
around this for built-in logical replication, but that was hack.

This is a more proper fix: We mark such transient heaps using the new
field pg_class.relwrite, linking to the original relation OID.  By
default, we ignore them in logical decoding before they get to the
output plugin.  Optionally, a plugin can register their interest in
getting such changes, if they handle DDL specially, in which case the
new field will help them get information about the actual table.

Reviewed-by: Craig Ringer <craig@2ndquadrant.com>

src/backend/commands/cluster.c

@@ -692,6 +692,7 @@ make_new_heap(Oid OIDOldHeap, Oid NewTableSpace, char relpersistence,
 										  false,
 										  true,
 										  true,
+										  OIDOldHeap,
 										  NULL);
 	Assert(OIDNewHeap != InvalidOid);