Fix a bug that Prev() can hang.

Summary: Prev() now can hang when there is a key with more than max_skipped number of appearance internally but all of them are newer than the sequence ID to seek. Add unit tests to confirm the bug and fix it.

Test Plan: make all check

Reviewers: igor, haobo

Reviewed By: igor

CC: ljin, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16899