Summary:
Right now, CompactionPri = kMinOverlappingRatio provides best write amplification, but it doesn't
prioritize files with more tombstones. We combine the two good features: make kMinOverlappingRatio
to boost files with lots of tombstones too.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4907

Differential Revision: D13788774

Pulled By: siying

fbshipit-source-id: 1991cbb495fb76c8b529de69896e38d81ed9d9b3