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:
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...
```

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

Inizia leggendo ContinueProcessOperation#run, ContinueProcessOperation#continueThroughFlowNode e ContinueProcessOperation#executeProcessStartExecutionListeners. Riproduci il problema con un processo eseguibile minimo contenente un async startEvent e un listener di esecuzione di avvio a livello di processo, quindi confrontalo con un evento di avvio sincrono. Il lavoro è completato quando il listener viene eseguito esattamente una volta all’avvio del processo async.

Scritto dal modello di indicizzazione a partire dal testo della issue.