Summary: WriteBatch and WriteBatchWithIndex now both inherit from a common abstract base class.  This makes it easier to write code that is agnostic toward the implementation of the particular write batch.  In particular, I plan on utilizing this abstraction to allow transactions to support using either implementation of a write batch.

Test Plan: modified existing WriteBatchWithIndex tests to test new functions.  Running all tests.

Reviewers: igor, rven, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34017