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/routing/RouterViaProgramDocSpec.scala

@@ -40,7 +40,7 @@ class RouterViaProgramDocSpec extends AkkaSpec with ImplicitSender {
     val actor3 = system.actorOf(Props[ExampleActor1], "actor3")
     val routees = Vector[String]("/user/actor1", "/user/actor2", "/user/actor3")
     val router = system.actorOf(
-      Props().withRouter(RoundRobinRouter(routees = routees)))
+      Props.empty.withRouter(RoundRobinRouter(routees = routees)))
     //#programmaticRoutingRouteePaths
     1 to 6 foreach { i ⇒ router ! Message1(i) }
     val received = receiveN(6, 5.seconds.dilated)