PersistentFSMActor: Remove duplication between akka.actor.FSM and akka.persistence.fsm.FSM

Currently akka.persistence.fsm.PersistentFSMActor inherits from akka.persistence.fsm.FSM - a clone of the original akka.actor.FSM with some Event Sourcing related changes in FSM's DSL. Possible ways to fix this: akka.persistence.fsm.FSM should either be derived from the original FSM or a common trait should be introduced - for example:

trait FSMCore {...}
trait FSM extends FSMCore {...}
trait EventSourcedFSM extends FSMCore {...}