RFR: 8342214: GenShen: Reduce code duplication in shFreeSet with iterator abstraction

[William Kemper]

These were almost all clean. All pipeline tests have passed.

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Commit messages:
 - 8342734: GenShen: Test failure gc/shenandoah/TestReferenceRefersToShenandoah.java#generational
 - 8344985: GenShen: Refactor arraycopy barrier for generational mode
 - 8344797: GenShen: Update and rename confusing method
 - 8344779: GenShen: Consolidate shared heap region closures into one header
 - 8344670: GenShen: Use concurrent worker session for concurrent mark phase
 - 8344640: GenShen: Reuse existing card mark barrier function when dropping references
 - 8344638: GenShen: Verifier should not touch claim token
 - 8344592: GenShen: Remove unnecessary comments and changes
 - 8344321: GenShen: Fix various sonar scan warnings
 - 8344320: GenShen: Possible null pointer usage in shGenerationalHeap
 - ... and 15 more: https://git.openjdk.org/shenandoah-jdk21u/compare/842a0d8b...1aed2ff5

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/143/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=143&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342214
  Stats: 950 lines in 59 files changed: 315 ins; 360 del; 275 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/143.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/143/head:pull/143

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/143