Properly destruct rebuilding_trx_

Summary:
When testing rebuilding_trx_ in MemTableInserter might still be set before the tests finishes which would cause ASAN alarms for leaks. This patch deletes the pointers in MemTableInserter destructor.
Closes https://github.com/facebook/rocksdb/pull/3162

Differential Revision: D6317113

Pulled By: maysamyabandeh

fbshipit-source-id: a68be70709a4fff7ac2b768660119311968f9c21

db/write_batch.cc

@@ -980,6 +980,7 @@ class MemTableInserter : public WriteBatch::Handler {
       reinterpret_cast<MemPostInfoMap*>
         (&mem_post_info_map_)->~MemPostInfoMap();
     }
+    delete rebuilding_trx_;
   }
 
   MemTableInserter(const MemTableInserter&) = delete;