[Short term] Implement retries and waiting for concurrent API calls in the case of throttling.

[pashpashpash]

I have had some people tell me that the GPT concurrent calls I am making right now are being throttled -- most likely because my Openai organization has higher concurrency limits compared to new accounts. This can be solved by

1. Limiting max concurrent requests
2. Retries should be implemented as well.

https://github.com/pashpashpash/manga-reader/blob/main/app.py#L47-L66

https://github.com/pashpashpash/manga-reader/blob/main/app.py#L173-L185

From someone who attempted to run the code:

openai.RateLimitError: Error code: 429 - {'error': {'message': 'Request too large for gpt-4-vision-preview in organization org-xxxxxxxxxx on tokens per min (TPM): Limit 10000, Requested 25012. The input or output tokens must be reduced in order to run successfully. Visit https://platform.openai.com/account/rate-limits to learn more.', 'type': 'tokens', 'param': None, 'code': 'rate_limit_exceeded'}}