Fix excessive Redis calls for new HSET/HMGET diffs cache

What does this MR do?

Before this change, we were submitting multiple HMGET calls to Redis unnecessarily when reading or writing to the cache, reaching incredibly high timings when enabling the hset_redis_diff_caching feature flag.

This MR fixes that by using a memoized method.

Closes #39133 (closed)

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Does this MR meet the acceptance criteria?

Conformity

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Security

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