- remove key_size attribute from all Locations
- now allow `-1` at the end of Location.levels
- add support for the local `stable_objects`, `unstable_objects`, `unstable_modules`
- add warning if s3 cannot update usage_date
- rename `is_stable`, `is_unstable`, `unstable_module` to `mark_stable`, `mark_unstable`, `mark_module_unstable` correspondingly
- `PickleKeyStorage` is using `storage` hash algorithm if `None`
- catching `zlib.error` in `NumpySerializer`