[15] RFR 8237963: Shenandoah: Heap iteration should use single-threaded string dedup oops_do_slow()

[Zhengyu Gu]

Please review this patch that uses single-threaded string dedup's 
oops_do() implementation for heap iteration.

The bug was reported by SAP on Windows, but it is not Windows specific. 
The bug is due to heap iteration uses parallel version of string dedup's 
oops_do() implementation, which can interfere concurrent string dedup 
cleaning task.


Bug: https://bugs.openjdk.java.net/browse/JDK-8237963
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8237963/webrev.00/index.html

Test:
    gc/shenandoah/jvmti/TestHeapDump.java test with 
-XX:+UseStringDeduplication (fastdebug and release) on x86_64 Linux.

Thanks,

-Zhengyu