[SPARK-4027][Streaming] HDFSBasedBlockRDD to read received either from BlockManager or WAL in HDFS

As part of the initiative of preventing data loss on streaming driver failure, this sub-task implements a BlockRDD that is backed by HDFS. This BlockRDD can either read data from the Spark's BlockManager, or read the data from file-segments in write ahead log in HDFS. 

Most of this code has been written by @harishreedharan

actually this discussion is moot because we should just let getFileSegmentLocations return Seq[String] rather than Option[Seq[String]], and then this should only consist of two branches, accomplishable with a single getOrElse.