[15] RFR 8241984: Shenandoah: enhance GCTimer and JFR support

[Zhengyu Gu]

There are a few enhancement to GC timer tracing and JFR support:

1) Setup for concurrent phase
2) Report sub phases to JFR
3) Report reference processing to JFR.


Bug: https://bugs.openjdk.java.net/browse/JDK-8241984
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8241984/webrev.00/

Test:
   hotspot_gc_shenandoah
   Eyeball GC events in JMC

Thanks,

-Zhengyu