Initial Merge of SPARK-20393 to 1.6 branch

Preparing Spark release v2.1.2-rc4

Preparing Spark release v2.1.2-rc3

Preparing Spark release v2.1.2-rc2

Preparing Spark release v2.1.2-rc1

Preparing Spark release v2.2.0-rc6

Preparing Spark release v2.1.1-rc4

Preparing Spark release v2.1.0-rc5

Preparing Spark release v2.0.2-rc3

Preparing Spark release v1.6.3-rc2