RFR: 8267257: Shenandoah: Always deduplicate strings when it is enabled during full gc

[Zhengyu Gu]

Full GC should recover as much memory as possible, so it should deduplicate all string candidates regardless their ages, when it is enabled. Also enhanced TestStringDedup.java test to ensure all candidates are deduplicated.

Note: Full GC does not preserve age bits, so current implementation will not deduplicate any strings even with back-to-back full GCs.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - Update test
 - Merge
 - Merge
 - v1
 - update
 - init

Changes: https://git.openjdk.java.net/jdk/pull/4096/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4096&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267257
  Stats: 140 lines in 9 files changed: 58 ins; 45 del; 37 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4096.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4096/head:pull/4096

PR: https://git.openjdk.java.net/jdk/pull/4096