RFR: 8220602: Shenandoah-SA: Enable best-effort implementation of heap walk

[Roman Kennke]

Hi Zhengyu,

> Please review this patch that provides best-effort implementation of 
> live regions iteration for Shenandoah GC.
> 
> There are minor changes in shared code, to adjust oop offset from base 
> of allocation cell.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8220602
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8220602/webrev.00/index.html
> 
> 
> Test:
>    hotspot_gc_shenandoah
>    hotspot_serviceability
>    vmTestbase/nsk/jdb
>    manual test: jhsdb hsdb
>    on Linux x64
> 
>    Passed submit test.

It looks mostly ok. But why do we need the change in CollectedHeap? Why 
not keep this an impl detail of Shenandoah heap?

Roman