=str add VirtualProcessor

- create a fully RS compliant minimal identity processor (that is not an
  Actor)
- replace SubscriberSourceVirtualProcessor,
  PublisherSinkVirtualSubscriber and PublisherSinkVirtualPublisher with
  this
- add tests that transform Sink.publisher’s and Source.subscriber’s
  materialized value
- also remove the Keep.{left, right} optimization in order to make
  side-effecting mat value transforms execute even if their values are
  discarded

akka-stream/src/main/scala/akka/stream/impl/Modules.scala

@@ -55,7 +55,7 @@ private[akka] abstract class SourceModule[+Out, +Mat](val shape: SourceShape[Out
 private[akka] final class SubscriberSource[Out](val attributes: OperationAttributes, shape: SourceShape[Out]) extends SourceModule[Out, Subscriber[Out]](shape) {
 
   override def create(context: MaterializationContext): (Publisher[Out], Subscriber[Out]) = {
-    val processor = new SubscriberSourceVirtualProcessor[Out]
+    val processor = new VirtualProcessor[Out]
     (processor, processor)
   }