feat: use async iterator to speedup http chunking

[LordOfPolls]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

This pr further speeds up guild chunking by leveraging the AsyncIterator to fetch members. This means the bot caches members as it fetches them, rather than fetching them all then caching. It also resolves the blocking problem as the async iterator wont block the event loop.

Overall this is only a ~10% speed improvement per chunk, but every little helps.

Changes

• Impliment member iterator
• Utitlise async iterator in chunking
• Add total_retrieved property to async iterator

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code