[11u] RFR 8251487: Shenandoah: missing detail timing tracking for final mark cleaning phase

[Zhengyu Gu]

Please review this small enhancement that adds detail timing tracking 
for final mark cleaning phase. The timing information is very useful to 
diagnose latency issues.

The patch is 11u specific, since 11u code structure is quite different 
from jdk/jdk, mainly due to concurrent class unloading in jdk/jdk.
The change is completed isolated in Shenandoah.


Bug: https://bugs.openjdk.java.net/browse/JDK-8251487
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8251487/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu