saneitizer: Fix shutdown when parallelism > 1

Then parallelism was > 1 flink would start multiple subtasks, but the structure of SanitySourceSplitEnumerator meant we only ever shutdown the task that was being used. Update to shutdown all subtask at the same time, rather than only the finished one.

If we ever want to use multiple splits this will have to be revisited, but it doesn't seem like that will be necessary.