RFR: 8350898: Shenandoah: Eliminate final roots safepoint [v4]

[William Kemper]

> This PR converts the final roots safepoint operation into a handshake. The safepoint operation still exists, but is only executed when `ShenandoahVerify` is enabled. In addition to this change, this PR also improves the logging for the concurrent preparation for update references from [PR 22688](https://github.com/openjdk/jdk/pull/22688).

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge remote-tracking branch 'jdk/master' into eliminate-final-roots
 - Clarify which thread local buffers in comment
 - Merge remote-tracking branch 'jdk/master' into eliminate-final-roots
 - Fix comments
 - Add whitespace at end of file
 - More detail for init update refs event message
 - Use timing tracker for timing verification
 - Merge remote-tracking branch 'jdk/master' into eliminate-final-roots
 - WIP: Fix up phase timings for newly concurrent final roots and init update refs
 - WIP: Combine satb transfer with state propagation, restore phase timing data
 - ... and 2 more: https://git.openjdk.org/jdk/compare/1dd9cf10...a3575f1e

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Changes: https://git.openjdk.org/jdk/pull/23830/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23830&range=03
  Stats: 291 lines in 14 files changed: 194 ins; 47 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/23830.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23830/head:pull/23830

PR: https://git.openjdk.org/jdk/pull/23830