Results 16 comments of dorianzheng

Could add some UT to cover new added methods and classes

@mxdzs0612 Nice work! Do you have any performance number before and after applying this PR?

@Kerollmops Mostly because the `DBIterator` won't position at any key before user calls one of `Seek`, `SeekForPrev`, `SeekToFirst`, or `SeekToLast` manually. Thank you for informing this, do you think we...