Orchestrator Java API should use `AbstractActor` with Akka 2.5

[anilgursel]

With Akka 2.5 upgrade, AbstractActor was improved and became more Java friendly. It now requires implementers to override a createReceive method. AbstractActor extends Actor and overrides receive from Actor, and just calls createReceive. This introduces a conflict in inheritance hierarchy of AbstractOrchestrator as both Aggregator and AbstractActor implements receive.

To unblock Akka 2.5 upgrade, AbstractActor was changed to inherit from Actor instead of AbstractActor, which makes AbstractOrchestrator less Java friendly.

In addition, Aggregator is deprecated in Akka 2.5.