Summary:
`WriteBatchWithIndex` has an incorrect implicitly-generated move constructor (it will copy the pointer causing a double-free on destruction). Just switch to `unique_ptr` so we get correct move semantics for free.
Closes https://github.com/facebook/rocksdb/pull/1899
Differential Revision: D4598896
Pulled By: ajkr
fbshipit-source-id: 2373d47