Summary:
Update HISTORY.md and version.h to 6.10.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6797

Reviewed By: zhichao-cao

Differential Revision: D21371390

Pulled By: anand1976

fbshipit-source-id: 6017bca24fc5d12076d1ddaec7783c9b85712d42