RFR(S) 8220444: Shenandoah should use parallel version of WeakProcessor in root processor for weak roots

[Roman Kennke]

Very nice!

Does the weakprocessor only handle the jni weak oops? I seem to remember 
it covers more 'weak roots' too?

Roman

> Shenandoah still uses serial version, should switch to parallel version.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8220444
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8220444/webrev.00/
> 
> Test:
>    hotspot_gc_shenandoah (fastdebug and release)
> 
>    Benchmark:
>    Compiler.compiler
>    Baseline: http://cr.openjdk.java.net/~zgu/JDK-8220444/serial.log
>    After Patch: http://cr.openjdk.java.net/~zgu/JDK-8220444/par.log
> 
> 
> Thanks,
> 
> -Zhengyu