IOStatisticsContext tuning

Tuning of the IOStatisticsContext code

change property name to fs.iostatistics....

there are other fs.iostatistics options, the new one needs consistent naming

enable in hadoop-aws

edit core-site.xml in hadoop-aws/test/resources to always collect context iOStatistics

This helps qualify the code

	  <property>
	    <name>fs.thread.level.iostatistics.enabled</name>
	    <value>true</value>
	  </property>

IOStatisticsContext to add add static probe to see if it is enabled.

lets apps know not to bother collecting/reporting