RFR: 8260998: Shenandoah: Restore reference processing statistics reporting

[Zhengyu Gu]

Please review this patch that restores reporting of reference processing statistics after JDK-8254315

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Commit messages:
 - JDK-8260998-ref-proc-stats

Changes: https://git.openjdk.java.net/jdk/pull/2362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260998
  Stats: 20 lines in 3 files changed: 18 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2362.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2362/head:pull/2362

PR: https://git.openjdk.java.net/jdk/pull/2362