RFR: 8350898: Shenandoah: Eliminate final roots safepoint [v3]

[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 incrementally with one additional commit since the last revision:

  Clarify which thread local buffers in comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23830/files
  - new: https://git.openjdk.org/jdk/pull/23830/files/0b2675af..390de7f9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23830&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23830&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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