Async StartEvent causes process level 'start' execution listener to be run twice

**Describe the bug**
If the start event element of the diagram is **async** then `ContinueProcessOperation#run` is called twice for the start event element which leads to calling any process level execution listeners twice as well (see also `ContinueProcessOperation#continueThroughFlowNode` and `ContinueProcessOperation#executeProcessStartExecutionListeners`).

If the start event element is **sync** then the listener is called just once (as expected).

**Expected behavior**
The process level start execution listener should be called just once when the process is started.

**Code**
Should be pretty straightforward for you to reproduce with any trivial diagram that starts like this:
```





...
```

**Additional context**
Using latest Flowable version (process engine only) with Spring Boot, Postgres 10.

Commencez par lire ContinueProcessOperation#run, ContinueProcessOperation#continueThroughFlowNode et ContinueProcessOperation#executeProcessStartExecutionListeners. Reproduisez le problème avec un processus exécutable minimal contenant un async startEvent et un listener d’exécution de démarrage au niveau du processus, puis comparez-le à un événement de démarrage synchrone. Le travail est terminé lorsque le listener s’exécute exactement une fois au démarrage du processus async.

Rédigé par le modèle d'indexation à partir du texte de l'issue.