Refactor RRDD by separating the common logic interacting with the R worker to a new class RRunner, which can be used to evaluate R UDFs.

Now RRDD relies on RRuner for RDD computation and RRDD could be reomved if we want to remove RDD API in SparkR later.

Author: Sun Rui <rui.sun@intel.com>

Closes #10947 from sun-rui/SPARK-12792.