Remove infinite caches to prevent memory leak

What does this PR do? (Please give us a brief description of what this PR does.)

This PR resolves #600 (closed) and #601 (closed) by replacing all "infinite" cache lifetimes and cleaning up MemoryKVCache<T>. Expiration lifetimes of all memory / redis caches have been reset to a reasonable number and normalized between similar caches. Redis caches have a synchronization fix, and an optimization is applied to MemoryKVCache.gc().

This is a smaller, more narrow-focused PR that replaces !580 (closed). A similar PR is open for Misskey: https://github.com/misskey-dev/misskey/pull/14363

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Me waiting on the commit with the timeout comment fixes before approving 🙂

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• 9930c64f - Fix timeout comments

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