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-actor-tests/src/test/scala/akka/testkit/CallingThreadDispatcherModelSpec.scala

@@ -32,7 +32,7 @@ object CallingThreadDispatcherModelSpec {
     extends MessageDispatcherConfigurator(config, prerequisites) {
 
     private val instance: MessageDispatcher =
-      new CallingThreadDispatcher(prerequisites, UnboundedMailbox(), false) with MessageDispatcherInterceptor {
+      new CallingThreadDispatcher(this) with MessageDispatcherInterceptor {
         override def id: String = config.getString("id")
       }