[15] RFR 8245961: Shenandoah: move some root marking to concurrent phase

[Zhengyu Gu]

Please review this patch that moves following root scanning to 
concurrent phase.

1) JNIHandle roots
2) VM global roots
3) CLD roots

and also consolidates code cache root marking.


Bug: https://bugs.openjdk.java.net/browse/JDK-8245961
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8245961/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu