[SPARK-1712]: TaskDescription instance is too big causes Spark to hang

Author: witgo <witgo@qq.com>

Closes #694 from witgo/SPARK-1712_new and squashes the following commits:

0f52483 [witgo] review commit
83ce29b [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
52e6752 [witgo] reset test SparkContext
63636b6 [witgo] review commit
44a59ee [witgo] review commit
3b6d48c [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
926bd6a [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
9a5cfad [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
03cc562 [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
b0930b0 [witgo] review commit
b1174bd [witgo] merge master
f76679b [witgo] merge master
689495d [witgo] fix scala style bug
1d35c3c [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
062c182 [witgo] fix small bug for code style
0a428cf [witgo] add unit tests
158b2dc [witgo] review commit
4afe71d [witgo] review commit
9e4ffa7 [witgo] review commit
1d35c7d [witgo] fix hang
7965580 [witgo] fix Statement order
0e29eac [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
3ea1ca1 [witgo] remove duplicate serialize
743a7ad [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
86e2048 [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
2a89adc [witgo] SPARK-1712: TaskDescription instance is too big causes Spark to hang

(cherry picked from commit 4dbb27b0cf4eb67c92aad2c1158616312f5a54e6)
Signed-off-by: Matei Zaharia <matei@databricks.com>