Summary:
Make "unreleased" section for HISTORY.md with things misplaced
into 6.12 and 6.13
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7401
Test Plan: see how it goes, and `git diff origin/6.13.fb HISTORY.md`
Reviewed By: jay-zhuang
Differential Revision: D23759740
Pulled By: pdillinger
fbshipit-source-id: fc441916c7ff2bbb8d5384137653b340d4c47674