RFR(S) 8223951: Shenandoah: Only need to update thread roots during final update refs

[Zhengyu Gu]

After switching to strong to-space invariant and pre-evacuating all 
root, thread roots are only places that may contain to-space oops after 
updating references during normal concurrent cycles, we only need to 
update them during final updaterefs pause, which should reduce pause time.

Bug: https://bugs.openjdk.java.net/browse/JDK-8223951
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8223951/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu