[SPARK-4467] fix elements read count for ExtrenalSorter

the elementsRead variable should be reset to 0 after each spilling

Author: Tianshuo Deng <tdeng@twitter.com>

Closes #3302 from tsdeng/fix_external_sorter_record_count and squashes the following commits:

7b56ca0 [Tianshuo Deng] fix method signature
782c7de [Tianshuo Deng] make elementsRead private, fix comment
bb7ff28 [Tianshuo Deng] update elemetsRead through addElementsRead method
74ca246 [Tianshuo Deng] fix elements read count
[SPARK-4470] Validate number of threads in local mode

When running Spark locally, if number of threads is specified as 0 (e.g., `spark-submit --master local[0] ...`), the job got stuck and does not run at all.
I think it's better to validate the parameter.

Fix for [SPARK-4470](https://issues.apache.org/jira/browse/SPARK-4470).

Author: Kenichi Maehashi <webmaster@kenichimaehashi.com>

Closes #3337 from kmaehashi/spark-4470 and squashes the following commits:

3ad76f3 [Kenichi Maehashi] fix code style
7716734 [Kenichi Maehashi] SPARK-4470: Validate number of threads in local mode
[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-4478] Keep totalRegisteredExecutors up-to-date

totalRegisteredExecutors keeps track of the number of executors
currently registered with the scheduler. This commit fixes SPARK-4478
bug by reducing the count when an executor is lost.
Added comment