Expose the serialized string that represents a WriteBatch.

Summary:
Expose the serialized string that represents a WriteBatch. This
is helpful to replicate a writebatch operation from one machine
to another.

Test Plan: make clean check

Reviewers: sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7317

include/leveldb/write_batch.h

@@ -51,6 +51,9 @@ class WriteBatch {
   };
   Status Iterate(Handler* handler) const;
 
+  // Returns the serialized string
+  std::string Data() { return rep_; }
+
  private:
   friend class WriteBatchInternal;