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

[Roman Kennke]

>> 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/
> 
> Looks fine to me.
> 
> Aside: I wonder how would be get the thread root with from-space ptr if we don't have safepoints
> between loads and the LRBs that give us to-space ptrs after potential evac.

I don't think we cann guarantee that we don't get a safepoint between
load and LRB.

Roman