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.
this name cannot change to `hsplit` - in scala parameter names are part of the binary API, so you can't change them.