Upgrade akka to 2.4.1+

Akka 2.4.x offers significant benefits in terms of performance and integration with Java 8. More importantly akka-persistence is no longer experimental with a good forward-compatibility story.

Also this release marks the introduction of Akka Typed, which is behavior-oriented take on actor interactions, which will allow us to implement RAFT and Shards in terms of Behaviors. These can be properly unit-tested with repeatable results and without having to resort to hacking actor systems.