[SPARK-4719][API] Consolidate various narrow dep RDD classes with MapPartitionsRDD

MappedRDD, MappedValuesRDD, FlatMappedValuesRDD, FilteredRDD, GlommedRDD, FlatMappedRDD are not necessary. They can be implemented trivially using MapPartitionsRDD.

Author: Reynold Xin <rxin@databricks.com>

Closes #3578 from rxin/SPARK-4719 and squashes the following commits:

eed9853 [Reynold Xin] Preserve partitioning for filter.
eb1a89b [Reynold Xin] [SPARK-4719][API] Consolidate various narrow dep RDD classes with MapPartitionsRDD.