RFR: Put regulator thread stop after gc thread iteration is prevented

[William Kemper]

Record that shutdown sequence has begun before we request the regulator thread to stop.

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Commit messages:
 - Put regulator thread stop after we block iteration of gc threads

Changes: https://git.openjdk.org/shenandoah/pull/231/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=231&range=00
  Stats: 6 lines in 1 file changed: 3 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/231.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/231/head:pull/231

PR: https://git.openjdk.org/shenandoah/pull/231