Summary:
In DBIter, Prev() calls FindValueForCurrentKey() to search the current value backward. If it finds that there are too many stale value being skipped, it falls back to FindValueForCurrentKeyUsingSeek(), seeking directly to the key with snapshot sequence. After introducing read_callback, however, the key it seeks to might not be visible, according to read_callback. It thus needs to keep searching forward until the first visible value.
Closes https://github.com/facebook/rocksdb/pull/3382

Differential Revision: D6756148

Pulled By: yiwu-arbug

fbshipit-source-id: 064e39b1eec5e083af1c10142600f26d1d2697be