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.

Start by reading ContinueProcessOperation#run, ContinueProcessOperation#continueThroughFlowNode, and ContinueProcessOperation#executeProcessStartExecutionListeners. Reproduce the issue with a minimal executable process containing an async startEvent and a process-level start execution listener, then compare it with a synchronous start event. Done means the listener runs exactly once when the async process starts.

Written by the indexing model from the issue text.