Make WriteBatchWithIndex moveble

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

include/rocksdb/utilities/write_batch_with_index.h

@@ -12,6 +12,7 @@
 
 #ifndef ROCKSDB_LITE
 
+#include <memory>
 #include <string>
 
 #include "rocksdb/comparator.h"
@@ -205,7 +206,7 @@ class WriteBatchWithIndex : public WriteBatchBase {
 
  private:
   struct Rep;
-  Rep* rep;
+  std::unique_ptr<Rep> rep;
 };
 
 }  // namespace rocksdb

utilities/write_batch_with_index/write_batch_with_index.cc

@@ -570,7 +570,7 @@ WriteBatchWithIndex::WriteBatchWithIndex(
     bool overwrite_key)
     : rep(new Rep(default_index_comparator, reserved_bytes, overwrite_key)) {}
 
-WriteBatchWithIndex::~WriteBatchWithIndex() { delete rep; }
+WriteBatchWithIndex::~WriteBatchWithIndex() {}
 
 WriteBatch* WriteBatchWithIndex::GetWriteBatch() { return &rep->write_batch; }