RFR: 8321605: GenShen: Old generation reference process is never reset

[William Kemper]

Shenandoah's reference processor workers keep state in thread local variables that need to be `reset` during init mark. Prior to this change, this was not happening for the old generation - resulting in an accumulation of counters.

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Commit messages:
 - Reset old gen reference processor thread locals when bootstrapping old marking

Changes: https://git.openjdk.org/shenandoah/pull/367/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=367&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321605
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/367.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/367/head:pull/367

PR: https://git.openjdk.org/shenandoah/pull/367