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.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #2087 from sryza/sandy-spark-2621 and squashes the following commits:

23010b8 [Sandy Ryza] Missing style fixes
74fc9bb [Sandy Ryza] Make getFSBytesReadOnThreadCallback private
1ab662d [Sandy Ryza] Clear things up a bit
984631f [Sandy Ryza] Switch from pull to push model and add test
7ef7b22 [Sandy Ryza] Add missing curly braces
219abc9 [Sandy Ryza] Fall back to split size
90dbc14 [Sandy Ryza] SPARK-2621. Update task InputMetrics incrementally