Updates the checkpoint data of the DStream. This gets called every time
the graph checkpoint is initiated. Default implementation records the
checkpoint files to which the generate RDDs of the DStream has been saved.
Cleanup old checkpoint data. This gets called after a checkpoint of time has been
written to the checkpoint directory.
public void restore()
Restore the checkpoint data. This gets called once when the DStream graph
(along with its DStreams) are being restored from a graph checkpoint file.
Default implementation restores the RDDs from their checkpoint files.