[Hot Fix #42] Persisted RDD disappears on storage page if re-used

If a previously persisted RDD is re-used, its information disappears from the Storage page.

This is because the tasks associated with re-using the RDD do not report the RDD's blocks as updated (they already exist in the BlockManager). On stage submit, however, we overwrite any existing information regarding that RDD with a fresh one, whether or not the information for the RDD already exists.