Fix BlockManagerUI bug by introducing new event

Previously, the storage information of persisted RDD's continued to rely on the old SparkContext,
which is no longer accessible if the UI is rendered from disk. This fix solves it by introducing
an event, SparkListenerGetRDDInfo, which captures this information.

Per discussion with Patrick, an alternative is to encapsulate this information within
SparkListenerTaskEnd. This would bypass the need to create a new event, but would also require
a non-trivial refactor of BlockManager / BlockStore.