[SPARK-1860] More conservative app directory cleanup.

First contribution to the project, so apologize for any significant errors.

This PR addresses [SPARK-1860]. The application directories are now cleaned up in a more conservative manner.

Previously, app-* directories were cleaned up if the directory's timestamp was older than a given time. However, the timestamp on a directory does not reflect the modification times of the files in that directory. Therefore, app-* directories were wiped out even if the files inside them were created recently and possibly being used by Executor tasks.

The solution is to change the cleanup logic to inspect all files within the app-* directory and only eliminate the app-* directory if all files in the directory are stale.

nit: I noticed that if you make the log message less grammatically correct and used string interpolation, you can fit it in exactly 100 characters.
```scala
logInfo(s"Worker cleanup enabled; old application directories will be deleted in: $workDir")
```