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.

What is going with the deltas here? The javadoc says it's the bytes read thus far, but if I look at the function here it seems like it will return to you a function that gives you the bytes read since the function was created.