[feature/performance] Store account stats in separate table

[tsmethurst]

Description

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This pull request changes the way we generate account stats. Previously, we were caching some stats in an in-memory cache. Before that, we just selected stats directly using COUNT queries every time an account needed to viewed via the API.

Instead in this PR stats are now stored in the database, and a best-effort is made to keep those stats up to date in the processing/worker functions.

This also means that we can now parse remote followers/following counts and store those, instead of having remote accounts always show counts only for local followers/following.

Stats for local accounts are regenerated if, when fetched from the cache/db, they haven't been regenerated for 24hrs or more, to avoid drift.

Some new functionality has been added to the cleaner package as well: every 24hrs at 1am a new "database cleaner" function will run. Currently this just regenerates stats for local accounts to prevent possible drift due to race conditions, but in future we could also use this new cleaner functionality to clean up old statuses when we get around to implementing that.

Should close #1887

Checklist

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[NyaaaWhatsUpDoc]

a few comments, and i need to spend some more time thinking about the client locks just to make sure there isn't an issue, but otherwise looking good so far!