Crash in db_bench if using custom environments and fillseqdeterministic

If using a custom environment in db_bench, the program may crash on exit. The problem is caused if we have an open environment *and* a database at exit. In this case, the environment is destroyed (via the std::shared_ptr env_guard) prior to the database being destroyed. The database may have some background jobs still running (for things like compaction) that dereference the environment that was just deleted.

To reproduce this, I was running db_bench against an AWS/RocksDB Cloud environment and running with --benchmark=fillseq,fillseqdeterministic.

Note that it does not happen if fillseqdeterministic is executed by itself (since the command exits before opening the database).