[SPARK-4027][Streaming] WriteAheadLogBackedBlockRDD to read received either...

[SPARK-4027][Streaming] WriteAheadLogBackedBlockRDD 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

Author: Tathagata Das <tathagata.das1565@gmail.com>
Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #2931 from tdas/driver-ha-rdd and squashes the following commits:

209e49c [Tathagata Das] Better fix to style issue.
4a5866f [Tathagata Das] Addressed one more comment.
ed5fbf0 [Tathagata Das] Minor updates.
b0a18b1 [Tathagata Das] Fixed import order.
20aa7c6 [Tathagata Das] Fixed more line length issues.
29aa099 [Tathagata Das] Fixed line length issues.
9e47b5b [Tathagata Das] Renamed class, simplified+added unit tests.
6e1bfb8 [Tathagata Das] Tweaks testuite to create spark contxt lazily to prevent contxt leaks.
9c86a61 [Tathagata Das] Merge pull request #22 from harishreedharan/driver-ha-rdd
2878c38 [Hari Shreedharan] Shutdown spark context after tests. Formatting/minor fixes
c709f2f [Tathagata Das] Merge pull request #21 from harishreedharan/driver-ha-rdd
5cce16f [Hari Shreedharan] Make sure getBlockLocations uses offset and length to find the blocks on HDFS
eadde56 [Tathagata Das] Transferred HDFSBackedBlockRDD for the driver-ha-working branch