[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>