Changed Failed to be a SystemMessage

- Moved system messages to their own package.
 - All queueing operations are now hidden behind a SystemMessageList value class
 - Introduced dual SystemMessageList types to encode the ordering in the type.
   - Protects against accidentally missed reverse calls or accidentally reversed lists
   - Makes ordering expectations by fields/parameters explicit
 - Fixed serialization tests
 - Fixes to logging in HierarchyStressSpec

akka-actor/src/main/scala/akka/dispatch/Dispatcher.scala

@@ -7,6 +7,7 @@ package akka.dispatch
 import akka.event.Logging.Error
 import akka.actor.ActorCell
 import akka.event.Logging
+import akka.dispatch.sysmsg.SystemMessage
 import java.util.concurrent.atomic.AtomicReference
 import java.util.concurrent.{ ExecutorService, RejectedExecutionException }
 import scala.concurrent.forkjoin.ForkJoinPool