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/replication/logical/reorderbuffer.c

@@ -1402,6 +1402,13 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 					if (!RelationIsLogicallyLogged(relation))
 						goto change_done;
 
+					/*
+					 * Ignore temporary heaps created during DDL unless the
+					 * plugin has asked for them.
+					 */
+					if (relation->rd_rel->relrewrite && !rb->output_rewrites)
+						goto change_done;
+
 					/*
 					 * For now ignore sequence changes entirely. Most of the
 					 * time they don't log changes using records we