RFR: SA Best-effort heap walking implementation

[Roman Kennke]

Very nice!

The patch looks ok. I wonder if we'd rather push it to upstream right 
away? It doesn't seem very risky, does it?

Thanks,
Roman

> This mirrors ZGC, provides best-effort heap walking implementation.
> 
> The patch applies cleanly for jdk/jdk and shenandoah/jdk, depending on
> if we want to bake for a while in shenandoah/jdk repo, or I can file
> upstream bug.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/sh-sa-heap-walk/webr
> ev.00/index.html
> 
> Test:
>    tier1, 2, 3_gc_shenandoah
>    hotspot_serviceability
>    vmTestbase/nsk/jdb
>    manual test: jhsdb hsdb
> 
> Thanks,
> 
> -Zhengyu
> 
>