Speed up db_bench shutdown

Summary: See t6489044

Test Plan: compiles

Reviewers: MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34977

db/db_bench.cc

@@ -1423,6 +1423,8 @@ class Benchmark {
                   [](ColumnFamilyHandle* cfh) { delete cfh; });
     delete db_.db;
     delete prefix_extractor_;
+    // this will leak, but we're shutting down so nobody cares
+    cache_->DisownData();
   }
 
   Slice AllocateKey(std::unique_ptr<const char[]>* key_guard) {