Java docs and sample of durable mailbox, see #2761

* Added abstract class DurableMessageQueueWithSerialization
(cherry picked from commit 49720e8cd09243ffea9f02c245e0053c126bf555)
Merge pull request #1160 from akka/wip-2761-java-durable-mailbox-master-patriknw

Java docs and sample of durable mailbox, see #2761