WriteBatchWithIndex's iterator to support SeekToFirst(), SeekToLast() and Prev()

Summary: Support SeekToFirst(), SeekToLast() and Prev() in WBWIIterator, returned by WriteBatchWithIndex::NewIterator().

Test Plan: Write unit test cases to cover the case.

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: rven, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D24765