add reentrancy

[qazwsxedckll]

Two remaining problems.

1. Users have to clone and copy protobuf/protoc-gen-go-grain/options/options.proto, in order to use options. grpc-gateway also uses this method.

2. Users have to unmarshal *cluster.GrainResponse to response type when using reenterancy. I cannot find a place to do this automatically.

               f, err := client.InvokeServiceFuture(&InvokeServiceRequest{Name: "Bob"})
       ctx.ReenterAfter(f, func(resp interface{}, err error) {
           if err != nil {
               onError(err)
               return
           }
   
           switch msg := resp.(type) {
           case *cluster.GrainResponse:
               result := &InvokeServiceResponse{}
               err = proto.Unmarshal(msg.MessageData, result)
               if err != nil {
                   onError(err)
               }
               respond(&InvokeServiceResponse{
                   Message: req.Name + " from " + ctx.Identity() + " and " + result.Message,
               })
           case *cluster.GrainErrorResponse:
               onError(errors.New(msg.Err))
           default:
               onError(errors.New("unknown response"))
           }
       })

[rogeralsing]

@qazwsxedckll , we could get rid of GrainResponse completely. see https://github.com/asynkron/protoactor-go/issues/994

it is just a wrapper of the result message and don´t really bring any value

[rogeralsing]

This looks fantastic!

[qazwsxedckll]

I suddenly realized that the respond and onError callback are not necessary. You can just call ctx.Respond() indise ctx.ReenterAfter(). I was just imitating the interface in dotnet.