Adding ability to override TimePathedSource step sizes.

We would like to be able to use MostRecentGoodSource on sources that are not split exactly on hour, day, month or year boundaries. For example, our batches are 8 hours and we want the most recent one.
 This is fine to merge, but note, you could have gotten this same behavior by overriding allPathsFor in your subclass. So, you are not blocked now, right?