BucketList cache

[SirTyson]

Description

Resolves #3696

This adds a persistent cache to BucketListDB at the bucket level. If a bucket is small, the entire bucket contents are cached in memory. For larger buckets, a random eviction cache is used instead.

Currently in draft form for max TPS tests, as I need to clean up the BucketIndex classes given that there's a fair bit of dead code now that caches are used.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document