Reimplement Distributed Entity Ownership Service on top of Akka Distributed Data

Our current implementation of Entity Ownership Service relies heavily on subclassing sal-distributed-datastore. This might have been kind-of-okay back in 2015, but it is making our life miserable due to the tight coupling here.

Akka has evolved quite a bit and now offers a number of cluster utilities, with Cluster Singleton and Distributed Data being the most prominent ones.

Implement EntityOwnershipService contract using these utilities instead of sal-distributed-datastore.