No longer serialize small indexes to disk

[SirTyson]

Description

Resolves #4268

Previously, we serialized every BucketListDB index to disk, regardless of size. This created significant disk churn in small, top level buckets, where new indexes are being re-written constantly.

This change no longer serializes the IndividualIndex type to disk to reduce this churn. By default, all buckets with size < 20 MB have individual indexes, so this will remove disk churn on the small, top most buckets. This also removes writing indexes to disk on the main thread, as previously Bucket::addBatch needed to write indexes to disk as part of ledgerClose.

There are essentially no drawbacks to recreating individual indexes on startup. They are so small that recreating them takes about as long as deserializing them. This change adds approximately 100ms to node startup time.

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

[dmkozh]

LGTM, let's merge after we unfreeze Core

[dmkozh]

r+ 66675ae