[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.

A bit off topic (and we can deal with this later) - but should we make the checkpoint directory into a `sparkConf` setting? That way we could do this type of validation earlier on. Right now unfortunately we can't distinguish here whether the user didn't call `checkpoint` or whether there was just a bug somewhere in Spark code.