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

[Roman Kennke]

Looks good to me!

Thank you!
Roman

> 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
>