[SPARK-4028][Streaming] ReceivedBlockHandler interface to abstract the functionality of storage of received data

As part of the initiative to prevent data loss on streaming driver failure, this JIRA tracks the subtask of implementing a ReceivedBlockHandler, that abstracts the functionality of storage of received data blocks. The default implementation will maintain the current behavior of storing the data into BlockManager. The optional implementation will store the data to both BlockManager as well as a write ahead log.

See my comment on that thread, on my preference to have Option in the name. Also spoke to @pwendell offline, and he does not seem to have a strong opinion on this.