[SPARK-3562]Periodic cleanup event logs

link #2391 
So, there's the potential that `info.lastUpdated` may be out of sync with the file system. This could happen if someone messes with the timestamps of the files, for example, so that `info.lastUpdated` is recent but the files in the directory were actually cleaned up by the code above.

In that case you'd have stale entries in the applications list and the user would see an error when clicking on the link.

That's not a huge issue, since it's not the only way that can happen (as with all cases where you're polling things). I was thinking about some way to make `checkForLogs` deal with that instead, but not sure if that would be any simpler.