Update to Akka 2.3.4 and add persistence

I noticed the scala 'activator-akka-distributed-workers' had been updated to 2.3.4 and included persistence, the java one hadn't and that was the one I was most interested in, as I knew no scala.

So I've had a stab at converting the java one to 2.3.4 with persistence, with the aim of learning akka and perhaps scala.
Although as it turns out I've think I've learnt more scala than Akka.

When running the code produces identical output to the Scala one, but the Tutorial section itself I haven't touched as I think thats probably best left to you guys.

hmm, there must be something missing in the Java API for ActorPath.fromString, I have created an issue for that: https://github.com/akka/akka/issues/15898

For now you can use a String parameter instead of ActorPath in startupSharedJournal.
actorSelection works fine with a String.