Summary:
Background jobs in WritePrepared DB might access the db via a snapshot checker callback. The stress tests therefore should cancel background jobs before deleting the db in ::Reopen.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6355

Differential Revision: D19664132

Pulled By: maysamyabandeh

fbshipit-source-id: 6060a830e8aad0015c10448286ad37c8a346ac01