RFR: 8320112 GenShen: Improve end of process report

[William Kemper]

When Shenandoah is shutdown, it logs a summary of its activity over the life of the process. This change adds percentages to the breakdown of the types and counts of cycles that executed. It also simplifies some of the logic around recording the various outcomes.

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Commit messages:
 - Cleanup, count abbreviated concurrent separately from abbreviated degen
 - WIP: It builds (after merging)
 - Merge remote-tracking branch 'shenandoah/master' into improve-end-of-process-report
 - WIP: Add percentages, stop creating gc session when filling old regions

Changes: https://git.openjdk.org/shenandoah/pull/357/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=357&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320112
  Stats: 173 lines in 14 files changed: 41 ins; 54 del; 78 mod
  Patch: https://git.openjdk.org/shenandoah/pull/357.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/357/head:pull/357

PR: https://git.openjdk.org/shenandoah/pull/357