SPARK-1767: Prefer HDFS-cached replicas when scheduling data-local tasks

This change reorders the replicas returned by
HadoopRDD#getPreferredLocations so that replicas cached by HDFS are at
the start of the list.  This requires Hadoop 2.5 or higher; previous
versions of Hadoop do not expose the information needed to determine
whether a replica is cached.
Yeah, I suppose there might be some confusion between the use of "new" here and the use of new in NewHadoopRdd / HadoopRdd.

Let's call it Hadoop2Classes.  That makes it fairly clear, right?