RFR: 8272327: Shenandoah: Avoid enqueuing duplicate string candidates

[Roman Kennke]

On Thu, 12 Aug 2021 00:12:14 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Now, there is a new API java_lang_String::test_and_set_deduplication_requested() that can help to identify if a string candidate has been enqueued, so that we can avoid enqueuing duplicate entries.
> 
> Test:
> - [x] hotspot_gc_shenandoah

Looks good to me, thanks!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5096