[Feat] New architectural proposal for the Statestore

[sleipnir]

Describe the feature
I propose using Nats as the primary source of persistence, or in other words as a source of storage for hot data. And using relational databases as a secondary source for storing cold data that has already expired ttl.

Why would that be important to you?
It allows you to more easily implement other types of flows such as Projections and collection types.

Additional context

Details: