[sh/jdk8u] RFR [backport] 8232747: Shenandoah: Concurrent GC should deactivate SATB before processing weak roots

[Roman Kennke]

> This is *not* straight forward backport, as sh/jdk8u has yet adopted 
> parallel_cleaning() from upstream, which is too risky to backport, IMHO.
> 
> The key point of this patch is that, we should deactivate SATB before 
> processing weak roots.
> 
> Original bug: https://bugs.openjdk.java.net/browse/JDK-8232747
> Original patch: https://hg.openjdk.java.net/jdk/jdk/rev/ef2b75750838
> 
> sh/jdk8u webrev: 
> http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8232747-8u/webrev.00/
> 
> Test:
>    hotspot_gc_shenandoah Linux x86_64 and x86_32
> 

Don't we also need:

+      set_has_forwarded_objects(false);

from the original patch?

Roman