Improve resource dispatcher mechanism

Following improvements were made:
 - a `ResourceDispatcher` now watches for death its managed actors, this
   enables to cleanup the maps when a managed actor dies,
 - A specific `Stop` message was added which, when sent to a dispatcher,
   is broadcast to all managed actors and then stops the dispatcher.
   This is necessary because the `PoisonPill` message is not sent to the
   `receive` method, even if it is queued as a standard message.
   Managing shutdown of the dispatcher in the `postStop` hook is not as
   flexible as having an explicit custom stop message. Managed actors
   can react to this message and do their own cleanup if necessary,
 - When someone asks to forward a message to some unknown resource, a
   hook method is called with the identifier of the requested unknown
   resource and the message that was meant to be forwarded. Dispatcher
   implementations can choose to deal with those messages as they wish
   (logging, specific answer, ...).