Add cache refresh rate

[tepinly]

Cache refresh rate

The main idea behind the PR is to introduce cache invalidation for outdated cache by referring to :cache_refresh_period for the max duration in minutes.

• Each hash record is stored with a cached_at element, indicating the time of caching
• When fetching said record, we check whether the time between cached_at and that of fetching is greater than :cache_refresh_period
  • If it is, the cache is invalidated & refreshed from the database
  • If it isn't, the cache is returned normally

Implementation progress

• Abstract the duration to a variable :cache_refresh_period
• Implement it in single fetch
• Implement it in multi fetch
• Support non-hash value types
• Add unit tests

[tepinly]

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