RFR: 8377713: Shenandoah: Convert ShenandoahReferenceProcessor to use Atomic<T> [v2]

[Ben Taylor]

> Modernizes the code as has been done in other areas of Shenandoah and G1 GCs.
> 
> The same tier1 tests pass before and after this change on x86_64 and aarch64 linux when run with -XX:+UseShenandoahGC

Ben Taylor has updated the pull request incrementally with one additional commit since the last revision:

  Change add_then_fetch - 1 to fetch_then_add

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29767/files
  - new: https://git.openjdk.org/jdk/pull/29767/files/f44b87cf..82d4de6c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29767&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29767&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29767.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29767/head:pull/29767

PR: https://git.openjdk.org/jdk/pull/29767