code: final <T> T actorProxyFor(final Class<T> protocol, final Actor actor, final Mailbox mailbox) {
      return ActorProxy.createFor(protocol, actor, mailbox);
    }
docs: Answers the T protocol proxy for this newly created Actor. (INTERNAL ONLY)
  @param <T> the protocol type
  @param protocol the {@code Class<T>} protocol of the Actor
  @param actor the Actor instance that backs the proxy protocol
  @param mailbox the Mailbox instance of this Actor
  @return T