Spring Easier policy setters for ThreadPoolTaskScheduler.

Added an easier way to set ExecuteExistingDelayedTasksAfterShutdown and ContinueExistingPeriodicTasksAfterShutdown policies. This closes an existing issue: https://github.com/spring-projects/spring-framework/issues/26719"

Comment From: pivotal-issuemaster

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Comment From: jhoeller

As per my comment on #26719, I ended up with a streamlined arrangement that does not include any policy getter methods.

Thanks for the pull request, in any case!