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/logical.c

@@ -317,6 +317,8 @@ CreateInitDecodingContext(char *plugin,
 		startup_cb_wrapper(ctx, &ctx->options, true);
 	MemoryContextSwitchTo(old_context);
 
+	ctx->reorder->output_rewrites = ctx->options.receive_rewrites;
+
 	return ctx;
 }
 
@@ -410,6 +412,8 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 		startup_cb_wrapper(ctx, &ctx->options, false);
 	MemoryContextSwitchTo(old_context);
 
+	ctx->reorder->output_rewrites = ctx->options.receive_rewrites;
+
 	ereport(LOG,
 			(errmsg("starting logical decoding for slot \"%s\"",
 					NameStr(slot->data.name)),