RFR: 8003209: JFR events for network utilization

[Robin Westberg]

Hi all,

Please review the following change with adds a JFR event for tracking network utilization on the network interface level.

Issue: https://bugs.openjdk.java.net/browse/JDK-8003209 <https://bugs.openjdk.java.net/browse/JDK-8003209>
Webrev: http://cr.openjdk.java.net/~rwestberg/8003209/webrev.00/ <http://cr.openjdk.java.net/~rwestberg/8003209/webrev.00/>
Testing: test/jdk/jdk/jfr/*

Many thanks to Erik Gahlin and Markus Grönlund who have looked at earlier versions of this change and suggested valuable improvements.

Best regards,
Robin