Unify NoSql and StoredDict Keys

[domenukk]

This (somewhat) addresses #3, by making sure keys are always stringified on set and retrieve.
It does not address non-string keys for nested dicts.

[ldruschk]

lgtm, although we should probably check how mypy deals with Union[NoSqlDict, StoredDict] and whether the type checks for __geitem__/__setitem__ emit a warning when using non-string keys

[domenukk]

The set and getitem calls for StoredDict always took str.
We can also change them both to always be of the Any type, since we now stringify them in any case.

[ldruschk]

I would rather let them be str and have a warning emitted if possible when using non-str keys