[15] RFR 8246591: Shenandoah: move string dedup roots scanning to concurrent phase

[Zhengyu Gu]

Please review this patch that moves string deduplication roots scanning 
to concurrent phase.

Bug: https://bugs.openjdk.java.net/browse/JDK-8246591
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8246591/webrev.00/

Test:
   hotspot_gc_shenandoah
   tier1 with "-XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions 
-XX:+ShenandoahVerify -XX:+UseStringDeduplication"

Thanks,

-Zhengyu