[15] RFR 8239354: Shenandoah: minor enhancements to traversal GC

[Zhengyu Gu]

1) Added assertion to catch evacuation after completion of heap 
traversal. This should help catch the bug demonstrated in sh-jdk11 w/o 
JDK-8237396.

2) Retire TLAB/GCLAB after completion of heap traversal. Current code 
retires TLAB/GCLAB at the beginning final traversal, but STW traversal 
still uses GCLAB to evacuate remaining objects.

3) Added comments regarding why need to retire TLAB/GCLAB, even we don't 
need heap to be parsable.


Bug: https://bugs.openjdk.java.net/browse/JDK-8239354
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8239354/webrev.00/index.html

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu