RFR: Put regulator thread stop after gc thread iteration is prevented [v2]

[William Kemper]

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

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

  Fix numbering for comments

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/231/files
  - new: https://git.openjdk.org/shenandoah/pull/231/files/2e174bec..d0138a4c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=231&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=231&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 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