tqdm in coscmd is not thread-safe

[xiacunshun]

When i try to download images with coscmd, it usually stuck. Then I add some output in the progress, it turns out that the program always stop at self._pbar.update(chunk_len). So I remove the tqdm, and it fixed.

I guess because the tqdm is not thread-safe, when multi-thread runs.

[xiacunshun]

[xiacunshun]

[lewzylu]

Thanks for your advice, I'll fix it soon