Liveset information for Old Object sample event

[Erik Gahlin]

Hi,

Could I have a review of an enhancement that adds heap usage after GC to 
the Old Object Sample (OOS) event. This is useful for spotting an 
increasing liveset.

Recordings typically only contain data for a limited period, i .e. last 
30 minutes, but the OOS event contains samples from when JFR/JVM was 
started, potentially several days back.

The liveset trend is useful for tools such as Mission Control to detect 
if there is a memory leak. If that is the case, information in the OOS 
event can be used to pinpoint where the leak occurred and what is 
keeping it alive. Presenting this information when there is not memory 
leak is confusing.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8197425

Webrev:
http://cr.openjdk.java.net/~egahlin/8197425/

Testings:
Tests in test/jdk/jdk/jfr

Thanks
Erik