RFR(XS) 8223215: Shenandoah: Support verifying subset of roots

[Roman Kennke]

Hi Zhengyu,

this looks good.
Maybe the root-verifier code should be in its own files?
What's with the font it looks weird? ;-)

Roman

> Please review this patch that allows to verify subset of roots.
> 
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8223215
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8223215/webrev.00/index.html
> 
> Test:
>   hotspot_gc_shenandoah with -XX:+ShenandoahVerify (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu