RFR: 8272327: Shenandoah: Avoid enqueuing duplicate string candidates

[Zhengyu Gu]

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

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Commit messages:
 - 8272327: Shenandoah: Avoid enqueuing duplicate string candidates

Changes: https://git.openjdk.java.net/jdk/pull/5096/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5096&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272327
  Stats: 10 lines in 3 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5096.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5096/head:pull/5096

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