[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

Getting LocalSparkContext is actually sort of painful as it is a test class, so we need to create a test-jar in mvn, which sbt does not pull in causing compilation failures even if the test-jar is added to the pom.