[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

> we don't want to get the HDFS locations if we know that the sparkPreferredLocations exists

The snippet that I listed actually implements this behavior: if sparkPreferreredLocations is not `None`, then the `orElse` short-circuits and never evaluates `hdfsPreferredLocations`.