SPARK-2621. Update task InputMetrics incrementally

The patch takes advantage an API provided in Hadoop 2.5 that allows getting accurate data on Hadoop FileSystem bytes read.  It eliminates the old method, which naively accepts the split size as the input bytes.  An impact of this change will be that input metrics go away when using against Hadoop versions earlier thatn 2.5.  I can add this back in, but my opinion is that no metrics are better than inaccurate metrics.

This is difficult to write a test for because we don't usually build against a version of Hadoop that contains the function we need.  I've tested it manually on a pseudo-distributed cluster.

Can you make this into a class constant?