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

[Zhengyu Gu]

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