Follow up `use_async` option

[Alnusjaponica]

#104 introduced asynchronous API calls of OpenAI-based evaluator and there are some follow-up tasks:

• Skip API call at the second time (calculation of scores) when unstructured assessment failed.
• Add test for use_async option
• Show progress bar when using async call.

[Alnusjaponica]

Use https://github.com/MagicStack/uvloop for further acceleration