WriteBatchWithIndex supports an iterator that merge its change with a base iterator.

Summary: Add an iterator that combines base_iterator of type Iterator* with delta iterator of type WBWIIterator*.

Test Plan: nothing yet. work in progress

Reviewers: ljin, igor

Reviewed By: igor

Subscribers: rven, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D24741