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

[Zhengyu Gu]

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.


Thanks,

-Zhengyu