RFR: 8336914: Shenandoah: Missing verification steps after JDK-8255765

[Satyen Subramaniam]

Adding before-update-refs verification step which was removed in previous revision [JDK-8255765](https://bugs.openjdk.org/browse/JDK-8255765) as directed by [JDK-8336914](https://bugs.openjdk.org/browse/JDK-8336914)

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Commit messages:
 - Setting update ref to in-progress after verification
 - Adding verify_before_updaterefs() to concurrent op_init_updaterefs()

Changes: https://git.openjdk.org/jdk/pull/20364/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20364&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336914
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20364.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20364/head:pull/20364

PR: https://git.openjdk.org/jdk/pull/20364