AsyncTaskInvokerTaskExecutor supplied by Spring Boot autoconfiguration is not started

[svenrienstra]

Describe the bug
When using Spring Boot autoconfiguration a AsyncTaskExecutor bean named flowableAsyncTaskInvokerTaskExecutor is supplied by org.flowable.spring.boot.FlowableJobConfiguration. However for the DefaultAsyncTaskExecutor to work start needs to be explicitly called, otherwise the executorService is null. Without the executorService a NullPointerException will be thrown when the bean is used.

Expected behavior
The flowableAsyncTaskInvokerTaskExecutor bean provided by default should be started.

Code
See PR.

Additional context
Version 7.0.1