[SPARK-4761][SQL] Enables Kryo by default in Spark SQL Thrift server

Enables Kryo and disables reference tracking by default in Spark SQL Thrift server. Configurations explicitly defined by users in `spark-defaults.conf` are respected (the Thrift server is started by `spark-submit`, which handles configuration properties properly).

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Author: Cheng Lian <lian@databricks.com>

Closes #3621 from liancheng/kryo-by-default and squashes the following commits:

70c2775 [Cheng Lian] Enables Kryo by default in Spark SQL Thrift server

(cherry picked from commit 6f61e1f961826a6c9e98a66d10b271b7e3c7dd55)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>
Streaming doc : do you mean inadvertently?

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #3620 from CrazyJvm/streaming-foreachRDD and squashes the following commits:

b72886b [CrazyJvm] do you mean inadvertently?

(cherry picked from commit 6eb1b6f6204ea3c8083af3fb9cd990d9f3dac89d)
Signed-off-by: Reynold Xin <rxin@databricks.com>
[SPARK-3623][GraphX] GraphX should support the checkpoint operation

Author: GuoQiang Li <witgo@qq.com>

Closes #2631 from witgo/SPARK-3623 and squashes the following commits:

a70c500 [GuoQiang Li] Remove java related
4d1e249 [GuoQiang Li] Add comments
e682724 [GuoQiang Li] Graph should support the checkpoint operation

(cherry picked from commit e895e0cbecbbec1b412ff21321e57826d2d0a982)
Signed-off-by: Ankur Dave <ankurdave@gmail.com>