[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

Does it make sense to take the SerializableWritable as the argument in the constructor (as being done in #2935) or should we just take the hadoopConf and wrap it in the SerializableWritable once that is merged? We don't want to change the interface later.