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