Refactor store-gateway index cache to remove duplication

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Is your feature request related to a problem? Please describe.

The index cache in the store-gateway currently has an in-memory implementation and a memcached implementation. There's a lot of overlap between these two implementation. This complicates changes required for #3772 (though index cache won't be the first cached updated, it may be eventually updated to use memory pooling).

Describe the solution you'd like

The caching package in dskit has an abstraction so that differences between caching layers can be ignored, we should use it with respect to the index cache. Then we can implement an in-memory version of the Cache interface and remove other versions of the indexcache besides the one meant to work with the dskit Cache interface.

Describe alternatives you've considered

N/A