Cleanup multiple DBs after running db_bench in multi-DB mode (#7891)

Summary:
Currently, db_bench cleanup only deletes the main DB, if there's one.
Multiple DBs that are opened when --num_multi_db is specified are not
deleted, which can lead to crashes due to running compaction threads on
process exit.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7891

Test Plan: Run regression test

Reviewed By: jay-zhuang

Differential Revision: D26049914

Pulled By: anand1976

fbshipit-source-id: acef2821001ca5e208a96a6a273c724e56353316

tools/db_bench_tool.cc

@@ -2784,6 +2784,9 @@ class Benchmark {
 
   ~Benchmark() {
     db_.DeleteDBs();
+    for (auto db : multi_dbs_) {
+      db.DeleteDBs();
+    }
     delete prefix_extractor_;
     if (cache_.get() != nullptr) {
       // this will leak, but we're shutting down so nobody cares