[SPARK-4026][Streaming] Write ahead log management

As part of the effort to avoid data loss on Spark Streaming driver failure, we want to implement a write ahead log that can write received data to HDFS. This allows the received data to be persist across driver failures. So when the streaming driver is restarted, it can find and reprocess all the data that were received but not processed.

This was primarily implemented by @harishreedharan. This is still WIP, as he is going to improve the unitests by using HDFS mini cluster.

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

Closes #2882 from tdas/driver-ha-wal and squashes the following commits:

e4bee20 [Tathagata Das] Removed synchronized, Path.getFileSystem is threadsafe
55514e2 [Tathagata Das] Minor changes based on PR comments.
d29fddd [Tathagata Das] Merge pull request #20 from harishreedharan/driver-ha-wal
a317a4d [Hari Shreedharan] Directory deletion should not fail tests
9514dc8 [Tathagata Das] Added unit tests to test reading of corrupted data and other minor edits
3881706 [Tathagata Das] Merge pull request #19 from harishreedharan/driver-ha-wal
4705fff [Hari Shreedharan] Sort listed files by name. Use local files for WAL tests.
eb356ca [Tathagata Das] Merge pull request #18 from harishreedharan/driver-ha-wal
82ce56e [Hari Shreedharan] Fix file ordering issue in WALManager tests
5ff90ee [Hari Shreedharan] Fix tests to not ignore ordering and also assert all data is present
ef8db09 [Tathagata Das] Merge pull request #17 from harishreedharan/driver-ha-wal
7e40e56 [Hari Shreedharan] Restore old build directory after tests
587b876 [Hari Shreedharan] Fix broken test. Call getFileSystem only from synchronized method.
b4be0c1 [Hari Shreedharan] Remove unused method
edcbee1 [Hari Shreedharan] Tests reading and writing data using writers now use Minicluster.
5c70d1f [Hari Shreedharan] Remove underlying stream from the WALWriter.
4ab602a [Tathagata Das] Refactored write ahead stuff from streaming.storage to streaming.util
b06be2b [Tathagata Das] Adding missing license.
5182ffb [Hari Shreedharan] Added documentation
172358d [Tathagata Das] Pulled WriteAheadLog-related stuff from tdas/spark/tree/driver-ha-working