Summary:
This patch adds the ability for the user to add sequences of arbitrary data (blobs) to write batches. These blobs are saved to the log along with everything else in the write batch. You can add multiple blobs per WriteBatch and the ordering of blobs, puts, merges, and deletes are preserved.

Blobs are not saves to SST files. RocksDB ignores blobs in every way except for writing them to the log.

Before committing this patch, I need to add some test code. But I'm submitting it now so people can comment on the API.

Test Plan: make -j32 check

Reviewers: dhruba, haobo, vamsi

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12195