RFR: Use distinct "end of cycle" message for each Shenandoah pause [v2]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Move region aging into op_final_roots

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/270/files
  - new: https://git.openjdk.org/shenandoah/pull/270/files/088002da..1c097462

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=270&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=270&range=00-01

  Stats: 37 lines in 1 file changed: 19 ins; 18 del; 0 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