RFR: Cool down regulator after request to start GC is accepted

[William Kemper]

Have the regulator wait for the gc state to change after the control thread accepts the request to start a cycle.

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Commit messages:
 - Locker safepoint check needs to match lock
 - Cool down regulator after gc request is accepted

Changes: https://git.openjdk.java.net/shenandoah/pull/130/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=130&range=00
  Stats: 9 lines in 2 files changed: 8 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/130.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/130/head:pull/130

PR: https://git.openjdk.java.net/shenandoah/pull/130