WriteBatch to provide a way for user to query data size directly and only return constant reference of data in Data()

Summary:
WriteBatch::Data() now is easily to be misuse by users. Also, there is no cheap way for user of WriteBatch to know the data size accumulated. This patch fix the problem by:
(1) return a constant reference to Data() so it's obvious to caller what it means.
(2) add a function to return data size directly

Test Plan: make all check

Reviewers: haobo, igor, kailiu

Reviewed By: kailiu

CC: zshao, leveldb

Differential Revision: https://reviews.facebook.net/D15123