RFR: 8389: JMC must stop using finalizers

[Marcus Hirt]

Also fixing some shutdown behaviour.

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Commit messages:
 - Cleanup
 - 8389: Getting rid of finalizers, also minor fixes

Changes: https://git.openjdk.org/jmc/pull/653/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=653&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8389
  Stats: 138 lines in 5 files changed: 100 ins; 20 del; 18 mod
  Patch: https://git.openjdk.org/jmc/pull/653.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/653/head:pull/653

PR: https://git.openjdk.org/jmc/pull/653