[SPARK-2797] [SQL] SchemaRDDs don't support unpersist()

The cause is explained in https://issues.apache.org/jira/browse/SPARK-2797.

Author: Yin Huai <huai@cse.ohio-state.edu>

Closes #1745 from yhuai/SPARK-2797 and squashes the following commits:

7b1627d [Yin Huai] The unpersist method of the Scala RDD cannot be called without the input parameter (blocking) from PySpark.