Cherry-pick: Concurrent class unloading related changes

[Roman Kennke]

Ok
Roman

> I would like to cherry-pick following concurrent class unloading related
> changes to shenandoah/jdk:
> 
> JDK-8227755: Need more than 2 distinct CodeCache unloading cycles
> JDK-8227866: Shenandoah: Split weak root processing and class unloading
> in parallel cleaning task
> JDK-8227635: Shenandoah: SHBSA::load_at() needs to deal IN_NATIVE load
> JDK-8228532: Shenandoah: Implement SBSA::try_resolve_jobject_in_native()
> 
> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/conc-unloading-cherry-pick/webrev.00/
> 
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu
> 
>