Json Serialization Mixins fix

This PR fixes the following issues:

* Inflight requests could result in a conflict when the key is the same but the type is different.
* `GetObjectCreatedAt` did not include the type in the key.

## TODO:

- [x] Discuss if we really need `bool noTypePrefix = false`
- [x] make sure the type is not part of the key for SQLite backend