Allow EventsourcedProcessor to post-process events

- Closes #370 in which an EventsourcedProcessor was not able to define custom routing destination
  aggregate IDs for the events produced by `processEvent`
- Introduces method EventsourcedProcessor.postProcessDurableEvent: DurableEvent => DurableEvent
  which defaults to identity and allows manipulation of the DurableEvent instance created as a
  consequence of new events generated by `processEvent`

eventuate-core/src/main/scala/com/rbmhtechnology/eventuate/EventsourcedProcessor.scala

@@ -27,7 +27,6 @@ import com.typesafe.config.Config
 import scala.collection.immutable.Seq
 import scala.concurrent._
 import scala.concurrent.duration._
-import scala.util._
 
 private class EventsourcedProcessorSettings(config: Config) {
   val readTimeout =
@@ -98,6 +97,14 @@ trait EventsourcedProcessor extends EventsourcedWriter[Long, Long] with ActorLog
    */
   def processEvent: Process
 
+  /**
+    * Override to allow post-processing of DurableEvent instances wrapping events generated by `processEvent`.
+    *
+    * Amongst other things, this can e.g. be used to set different or additional aggregate IDs for custom routing
+    * destinations (which by default take the same routing destinations as the original event that was processed).
+    */
+  def postProcessDurableEvent: DurableEvent => DurableEvent = identity[DurableEvent]
+
   /**
    * Returns an command [[BehaviorContext]] that doesn't allow event handler behavior changes. An
    * attempt to change the event handler behavior with `eventContext.become()` will throw an
@@ -113,7 +120,9 @@ trait EventsourcedProcessor extends EventsourcedWriter[Long, Long] with ActorLog
     case payload if processEvent.isDefinedAt(payload) =>
       val currentProcessedEvents = processEvent(payload)
       if (lastSequenceNr > processingProgress)
-        processedEvents = processedEvents :+ currentProcessedEvents.map(createEvent(_, lastHandledEvent.customDestinationAggregateIds))
+        processedEvents = processedEvents :+ currentProcessedEvents.map { e =>
+          postProcessDurableEvent(createEvent(e, lastHandledEvent.customDestinationAggregateIds))
+        }
   }
 
   /**