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/pattern/AskSupport.scala

@@ -7,7 +7,7 @@ import language.implicitConversions
 
 import java.util.concurrent.TimeoutException
 import akka.actor._
-import akka.dispatch._
+import akka.dispatch.sysmsg._
 import scala.annotation.tailrec
 import scala.util.control.NonFatal
 import scala.concurrent.{ Future, Promise, ExecutionContext }