[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
[SPARK-4481][Streaming][Doc] Fix the wrong description of updateFunc

Removed `If `this` function returns None, then corresponding state key-value pair will be eliminated.` for the description of `updateFunc: (Iterator[(K, Seq[V], Option[S])]) => Iterator[(K, S)]`

Author: zsxwing <zsxwing@gmail.com>

Closes #3356 from zsxwing/SPARK-4481 and squashes the following commits:

76a9891 [zsxwing] Add a note that keys may be added or removed
0ebc42a [zsxwing] Fix the wrong description of updateFunc
[HOT FIX] MiMa tests are broken

This is blocking #3353 and other patches.

Author: Andrew Or <andrew@databricks.com>

Closes #3371 from andrewor14/mima-hot-fix and squashes the following commits:

842d059 [Andrew Or] Move excludes to the right section
c4d4f4e [Andrew Or] MIMA hot fix
SPARK-3962 Marked scope as provided for external projects.

Somehow maven shade plugin is set in infinite loop of creating effective pom.

Author: Prashant Sharma <prashant.s@imaginea.com>
Author: Prashant Sharma <scrapcodes@gmail.com>

Closes #2959 from ScrapCodes/SPARK-3962/scope-provided and squashes the following commits:

994d1d3 [Prashant Sharma] Fixed failing flume tests
270b4fb [Prashant Sharma] Removed most of the unused code.
bb3bbfd [Prashant Sharma] SPARK-3962 Marked scope as provided for external.
[DOC][PySpark][Streaming] Fix docstring for sphinx

This commit should be merged for 1.2 release.
cc tdas

Author: Ken Takagiwa <ugw.gi.world@gmail.com>

Closes #3311 from giwa/patch-3 and squashes the following commits:

ab474a8 [Ken Takagiwa] [DOC][PySpark][Streaming] Fix docstring for sphinx