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