new Prev() prefix support using SeekForPrev()
Summary: 1) The previous solution for Prev() prefix support is not clean. Since I add api SeekForPrev(), now the Prev() can be symmetric to Next(). and we do not need SeekToLast() to be called in Prev() any more. Also, Next() will Seek(prefix_seek_key_) to solve the problem of possible inconsistency between db_iter and merge_iter when there is merge_operator. And prefix_seek_key is only refreshed when change direction to forward. 2) This diff also solves the bug of Iterator::SeekToLast() with iterate_upper_bound_ with prefix extractor. add test cases for the above two cases. There are some tests for the SeekToLast() in Prev(), I will clean them later. Test Plan: make all check Reviewers: IslamAbdelRahman, andrewkr, yiwu, sdong Reviewed By: sdong Subscribers: andrewkr, dhruba, leveldb Differential Revision: https://reviews.facebook.net/D63933
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