Summary:
Added a `CompactionFilter` that is aware of the stress test's expected state. It only drops key versions that are already covered according to the expected state. It is incompatible with snapshots (same as all `CompactionFilter`s), so disables all snapshot-related features when used in the crash test.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6988

Test Plan:
running a minified blackbox crash test

```
$ TEST_TMPDIR=/dev/shm python tools/db_crashtest.py blackbox --max_key=1000000 -write_buffer_size=1048576 -max_bytes_for_level_base=4194304 -target_file_size_base=1048576 -value_size_mult=33 --interval=10 --duration=3600
```

Reviewed By: anand1976

Differential Revision: D22072888

Pulled By: ajkr

fbshipit-source-id: 727b9d7a90d5eab18be0ec6cd5a810712ac13320