[SPARK-12792] [SPARKR] Refactor RRDD to support R UDF.

## What changes were proposed in this pull request?

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.

## How was this patch tested?
dev/lint-r
SparkR unit tests

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

Closes #12024 from sun-rui/SPARK-12792_new.