BucketListDB in-memory Buckets

[SirTyson]

Description

Partially resolves #3696

This change refactors the BucketIndex parts of BucketListDB to be more friendly towards the Hot Archive BucketList. This includes cleanups to metrics, which previously only tracked metrics for the main thread access of the BucketList. Now, both BucketList types and background threads record metrics properly.

Additionally, this change removes the IndividualIndex and instead caches small Buckets entirely in-memory so we never read from disk. RangeIndex is largely unchanged but has been renamed to DiskIndex.

A follow up PR will add a random eviction cache to the DiskIndex. I tried to break this up as much as possible, but it was easiest to do the refactor + in-memory buckets at the same time so I didn't have to refactor IndividualIndex.

The BUCKETLIST_DB_INDEX_CUTOFF config setting determines the maximum size at which we keep bucket in-memory. I've set this to 250 MB, which is approximately the first 4-5 levels of the BucketList. This increases total memory consumption of stellar-core from 2.2 GB to 3 GB. This seems reasonable, and we could probably go even higher, but I'm holding off for now as the random eviction cache will further increase memory requirements.

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