code: public <T> T actorFor(final Class<T> protocol, final Definition definition) {
      if (isTerminated()) {
        throw new IllegalStateException("vlingo/actors: Stopped.");
      }
  
      return stage().actorFor(protocol, definition);
    }
docs: Answers the {@code T} protocol of the newly created {@code Actor} that is defined by
  the parameters of {@code definition} that implements the {@code protocol}.
  @param protocol the {@code Class<T>} protocol that the {@code Actor} supports
  @param <T> the protocol type
  @param definition the {@code Definition} providing parameters to the {@code Actor}
  @return T