refactor the async api call to lambda style

if (recursive) {
    ZKUtil.visitSubTreeDFS(zk, path, watch, new StringCallback() {
        @Override
        public void processResult(int rc, String path, Object ctx, String name) {
            out.println(path);
        }
    });
}

ZKUtil.visitSubTreeDFS(zk, path, watch, (rc, path1, ctx, name) -> out.println(path1));