[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 (which is correct). 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.

Author: Andrew Or <andrewor14@gmail.com>

Closes #281 from andrewor14/ui-storage-fix and squashes the following commits:

408585a [Andrew Or] Fix storage UI bug