Fix issue with iterator operations in this order: Prev(), Seek(), Prev()

Summary:
Due to a bad merge of D14163 and D14001 before checking in D14001, "direction_ = kForward;" in MergeIterator::Seek() was deleted my mistake (in commit b135d01e7bcdf4186ea852a5b4e6d14a3a815d77 ). It will generate wrong results or assert failure after the sequence of Prev() (or SeekToLast()), Seek() and Prev().

Fix it

Test Plan: make all check

Reviewers: igor, haobo, dhruba

Reviewed By: igor

CC: yhchiang, i.am.jin.lei, ljin, leveldb

Differential Revision: https://reviews.facebook.net/D16527