Send one-way commands/notifications to saga participants

[cer]

Requires: eventuate-tram/eventuate-tram-core#174

Motivation is the concurrent execution of compensating transactions and retriable transactions, which are saga steps that by definition cannot fail.

[cer]

Proposed design consists of additional Saga DSL methods:

• notifyParticipant()
• withCompensationNotification()

public class NotificationBasedCreateOrderSaga implements SimpleSaga<NotificationBasedCreateOrderSagaData> {

  private final SagaDefinition<NotificationBasedCreateOrderSagaData> sagaDefinition;

  public NotificationBasedCreateOrderSaga(NotificationBasedCreateOrderSagaSteps steps) {
    this.sagaDefinition =
            step()
                  .invokeLocal(steps::createOrder)
                  .withCompensation(steps::rejectOrder)
            .step()
                  .invokeParticipant(NotificationBasedCreateOrderSagaData::reserveCredit)
                  .withCompensation(NotificationBasedCreateOrderSagaData::releaseCredit)
            .step()
                    .invokeParticipant(NotificationBasedCreateOrderSagaData::reserveInventory)
                    .withCompensationNotification(NotificationBasedCreateOrderSagaData::releaseInventory)
            .step()
                  .invokeLocal(steps::approveOrder)
            .step()
                  .notifyParticipant(NotificationBasedCreateOrderSagaData::fulfillOrder)
            .build();
  }

[cer]

Various remaining TODOs - see // TODO notifications

• [] Reactive Sagas
• [] Endpoints that support notifications
• [] conditional notifications