[SPARK-4482][Streaming] Disable ReceivedBlockTracker's write ahead log by default

The write ahead log of ReceivedBlockTracker gets enabled as soon as checkpoint directory is set. This should not happen, as the WAL should be enabled only if the WAL is enabled in the Spark configuration.

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

Closes #3358 from tdas/SPARK-4482 and squashes the following commits:

b740136 [Tathagata Das] Fixed bug in ReceivedBlockTracker

(cherry picked from commit 22fc4e751c0a2f0ff39e42aa0a8fb9459d7412ec)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>