RFR: Use distinct "end of cycle" message for each Shenandoah pause

[William Kemper]

This change allows the `GarbageCollectionNotificationInfo::gcAction` to be overridden. Shenandoah now uses this to distinguish the notification raised for each of the pauses during the concurrent cycle. This will make it easier for monitoring software to identify which phases may be having longer than expected pauses.

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Commit messages:
 - Simplify region aging logic
 - Allow unique message for each gc notification action

Changes: https://git.openjdk.org/shenandoah/pull/270/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=270&range=00
  Stats: 88 lines in 12 files changed: 37 ins; 21 del; 30 mod
  Patch: https://git.openjdk.org/shenandoah/pull/270.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/270/head:pull/270

PR: https://git.openjdk.org/shenandoah/pull/270