[Tiered Caching] Add a memory-efficient key lookup store for use in tiered cache

[peteralfonsi]

Description

Adds a roaring bitmap-based key lookup store, designed to store integer hashcodes of keys for a future disk cache in a memory-efficient way. It stores values in the RBM with a modulo, as very sparse RBMs are not memory-efficient. This comes with a tradeoff of some rare collisions (~0.5% of values for the recommended modulo 2^28 in a store with 10^7 values). To handle this, we also maintain a hash set of values which have had collisions. Values with no collisions can be safely removed without risking any false negatives in contains(). The keystore has an optional memory cap and will not add more values once it grows too large. To enable this, we also had to make an RBM size estimator based on performance test data, since the built-in one is very inaccurate for randomly-distributed data like hashes:

This implementation is generic and can be used in other places as well.

We investigated different data structures (RBMs, sorted int[], hash sets, or a hybrid combination of all of these) for memory footprint and access time before settling on an RBM with this choice of modulo. While a sorted int[] can be more memory-efficient than an RBM when it contains less than about 50,000 values, it is much slower because it requires binary search to add or access elements. Hash sets are more memory efficient below about 5,000 values. However, we are mostly concerned with memory efficiency when the store contains many keys, so we decided against adding the complexity of switching between data structures. We expect to allocate 5% of the on-heap cache size to this keystore, which would allow many domains to store 2-10 million key hashes, based on a domain scan that looks for domains which will benefit from tiered caching:

Related Issues

Resolves #10309

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
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• [N/A] Commit changes are listed out in CHANGELOG.md file (See: Changelog)
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This PR is stalled because it has been open for 30 days with no activity.