[SPARK-7056] [STREAMING] Make the Write Ahead Log pluggable

Users may want the WAL data to be written to non-HDFS data storage systems. To allow that, we have to make the WAL pluggable. The following design doc outlines the plan.

https://docs.google.com/a/databricks.com/document/d/1A2XaOLRFzvIZSi18i_luNw5Rmm9j2j4AigktXxIYxmY/edit?usp=sharing

Things to add.
* Unit tests for WriteAheadLogUtils

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #5645 from tdas/wal-pluggable and squashes the following commits:

2c431fd [Tathagata Das] Minor fixes.
c2bc7384 [Tathagata Das] More changes based on PR comments.
569a416 [Tathagata Das] fixed long line
bde26b1 [Tathagata Das] Renamed segment to record handle everywhere
b65e155 [Tathagata Das] More changes based on PR comments.
d7cd15b [Tathagata Das] Fixed test
1a32a4b [Tathagata Das] Fixed test
e0d19fb [Tathagata Das] Fixed defaults
9310cbf [Tathagata Das] style fix.
86abcb1 [Tathagata Das] Refactored WriteAheadLogUtils, and consolidated all WAL related configuration into it.
84ce469 [Tathagata Das] Added unit test and fixed compilation error.
bce5e75 [Tathagata Das] Fixed long lines.
837c4f5 [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into wal-pluggable
754fbf8 [Tathagata Das] Added license and docs.
09bc6fe [Tathagata Das] Merge remote-tracking branch 'apache-github/master' into wal-pluggable
7dd2d4b [Tathagata Das] Added pluggable WriteAheadLog interface, and refactored all code along with it