WriteBatchWithIndex: a wrapper of WriteBatch, with a searchable index

Summary:
Add WriteBatchWithIndex so that a user can query data out of a WriteBatch, to support MongoDB's read-its-own-write.

WriteBatchWithIndex uses a skiplist to store the binary index. The index stores the offset of the entry in the write batch. When searching for a key, the key for the entry is read by read the entry from the write batch from the offset.

Define a new iterator class for querying data out of WriteBatchWithIndex. A user can create an iterator of the write batch for one column family, seek to a key and keep calling Next() to see next entries.

I will add more unit tests if people are OK about this API.

Test Plan:
make all check
Add unit tests.

Reviewers: yhchiang, igor, MarkCallaghan, ljin

Reviewed By: ljin

Subscribers: dhruba, leveldb, xjin

Differential Revision: https://reviews.facebook.net/D21381