implement and document Pipelines, see #3174

- heavily inspired by spray.io.Pipeline
- fully functional style: a stage returns the resulting commands and
  events, which makes it impossible to mess with the pipeline from the
  inside
- object allocations are optimized away for emtpy and 1-elem results
- added type-safety, verifying that stages match up
- management commands “from the side” for configuration or async events
- full Java API and docs

akka-multi-node-testkit/src/main/scala/akka/remote/testkit/MultiNodeSpec.scala

@@ -293,12 +293,12 @@ abstract class MultiNodeSpec(val myself: RoleName, _system: ActorSystem, _roles:
   /**
    * Override this method to do something when the whole test is starting up.
    */
-  protected def atStartup(): Unit = {}
+  protected def atStartup(): Unit = ()
 
   /**
    * Override this method to do something when the whole test is terminating.
    */
-  protected def afterTermination(): Unit = {}
+  protected def afterTermination(): Unit = ()
 
   /**
    * All registered roles