Summary:
Cleanup TTLExtractor interface. The original purpose of it is to allow our users keep using existing `Write()` interface but allow it to accept TTL via `TTLExtractor`. However the interface is confusing. Will replace it with something like `WriteWithTTL(batch, ttl)` in the future.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4229
Differential Revision: D9174390
Pulled By: yiwu-arbug
fbshipit-source-id: 68201703d784408b851336ab4dd9b84188245b2d