Turn on log_replication_commands in PostgresNode

This is useful for example for the pg_basebackup and related tests.

src/test/perl/PostgresNode.pm

@@ -419,6 +419,7 @@ sub init
 	print $conf "restart_after_crash = off\n";
 	print $conf "log_line_prefix = '%m [%p] %q%a '\n";
 	print $conf "log_statement = all\n";
+	print $conf "log_replication_commands = on\n";
 	print $conf "wal_retrieve_retry_interval = '500ms'\n";
 	print $conf "port = $port\n";