[Shenandoah 11u] RFR: Root processing related shenandoah-11u backports

[Zhengyu Gu]

Backport following CRs to Shenandoah 11u, also fixed an issue in early 
backport.

8227101: Shenandoah: Use ShenandoahSharedFlag for claimed flag in
          ShenandoahSerialRoot

8227866: Shenandoah: Split weak root processing and class unloading in
          parallel cleaning task

8236902: Shenandoah: Missing string dedup roots in all root scanner

8231999: Shenandoah: Traversal failed compiler/jsr292
         /CallSiteDepContextTest.java
   [partial]



Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh_11u_root_proc_backports/webrev.00/


Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu