[Spark-4060] [MLlib] exposing special rdd functions to the public

Author: Niklas Wilcke <1wilcke@informatik.uni-hamburg.de>

Closes #2907 from numbnut/master and squashes the following commits:

7f7c767 [Niklas Wilcke] [Spark-4060] [MLlib] exposing special rdd functions to the public, #2907

(cherry picked from commit f90ad5d426cb726079c490a9bb4b1100e2b4e602)
Signed-off-by: Xiangrui Meng <meng@databricks.com>