fix: The shutdown of the application was blocked

fix: The shutdown of the application was blocked

* The auto-configured bean `applicationTaskExecutor` must not block, while
  it is shutting down, because otherwise, it infinitly waits for the
  completion, of the channel-tasks, which are stopped in a _later_ phase
  of the "smart" lifecycle.
* The bean is destroyed first, becaus it is is associated with the lowest
  lifecyle-phase (``Integer.MAX_VALUE``), which apparently cannot be
  overruled by `@DependsOn` (although suggested by the spring-
  documentation)
* Joining the channel-tasks was blocking infinitly, because the tasks were
  waiting for the kafka-consumers to be closed, what only happens _after_
  the joining completes.