rewrite mailbox selection logic, see #3342

- add “mailbox-requirement” key to dispatcher section
- split out mailbox section, add akka.actor.default-mailbox
- rewrite findMarker method and use it for Props.create() and getting
  the required mailbox of an actor
- add ProducesMessageQueue trait for MailboxType so that requirements
  can be checked before trying to create the actor for real
- verify actor as well as dispatcher requirements for message queue
  before creation, even in remote-deployed case
- change MessageDispatcher constructor to take a Configurator, add that
  to migration guide

akka-docs/rst/scala/code/docs/dispatcher/DispatcherDocSpec.scala

@@ -203,8 +203,7 @@ object DispatcherDocSpec {
     import akka.dispatch.{
       Envelope,
       MessageQueue,
-      QueueBasedMessageQueue,
-      UnboundedMessageQueueSemantics
+      UnboundedQueueBasedMessageQueue
     }
 
     // This constructor signature must exist, it will be called by Akka
@@ -213,7 +212,7 @@ object DispatcherDocSpec {
     // The create method is called to create the MessageQueue
     final override def create(owner: Option[ActorRef],
                               system: Option[ActorSystem]): MessageQueue =
-      new QueueBasedMessageQueue with UnboundedMessageQueueSemantics {
+      new UnboundedQueueBasedMessageQueue {
         final val queue = new ConcurrentLinkedQueue[Envelope]()
       }
   }