RFR (M): JDK-8068394: Trace event for concurrent GC phases

[sangheon.kim]

Hi all,

Can I get some reviews for this change of adding a trace event for 
concurrent phases on CMS and G1?

Currently we only measure pause times for major phases.
But I want to add 'concurrent mark' and 'concurrent sweep' phases for 
CMS and 'concurrent mark' phase for G1.
To achieve this, I had to change ConcurrentGCTimer and related classes.

This patch includes:
1) A concurrent phase can be measured only from ConcurrentGCTimer and 
measuring an overlap between concurrent and pause is not allowed as 
currently we don't have that use case. And TimePartitions class(or 
related classes) will be simpler than an overlap allowed case.
2) I removed PausePhase and ConcurrentPhase which are derived from 
GCPhase because I wanted to avoid heap allocation when adding to 
GrowableArray. Instead introduced 'type' member variable at GCPhase.

CR: https://bugs.openjdk.java.net/browse/JDK-8068394
Webrev: http://cr.openjdk.java.net/~sangheki/8068394/webrev.00/
Testing: JPRT

Thanks,
Sangheon