RFR: SA Best-effort heap walking implementation

[zgu at redhat.com]

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