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.

首先阅读 ContinueProcessOperation#run、ContinueProcessOperation#continueThroughFlowNode 和 ContinueProcessOperation#executeProcessStartExecutionListeners。使用包含 async startEvent 和流程级 start execution listener 的最小可执行流程重现该问题，然后将其与同步 start event 进行比较。当 async 流程启动时 listener 恰好运行一次，即表示完成。

由索引模型根据 Issue 内容生成。