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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