RFR: 8332841: GenShen: Pull shared members from control thread into common base class

[William Kemper]

This change facilitates further extensions from this base class in the forthcoming generational mode for Shenandoah.

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Commit messages:
 - Pull shared members from control thread into common base class

Changes: https://git.openjdk.org/jdk/pull/19372/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19372&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332841
  Stats: 365 lines in 5 files changed: 224 ins; 129 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/19372.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19372/head:pull/19372

PR: https://git.openjdk.org/jdk/pull/19372