Persistence - Let `Relation`-impl handle it

[CodeLionX]

Issue

A database should provide a way to persist all data to disk. Currently ADBMS only holds data in-memory. We could use the Relation-classes to provide persistence:

• use akka' s PersistentActor for saving state to disk
• Dactor contains relations, which are themselves PersistentActors
• maybe interface: object User extends RowRelation with AkkaPersistence(config) { ... }
• application dev. can still declare transient relations via omitting the trait AkkaPersistence
• relations handle their persistence themselves

Problem Description

Actor State is not persisted to disk.

Supporting Information

• https://doc.akka.io/docs/akka/2.5.5/scala/persistence.html
• If we use akka-cluster and akka-cluster-sharding, we must use a distributed journal, like Cassandra, Hazelcast, DynamoDB (https://index.scala-lang.org/search?topics=akka-persistence)