RFR: Use active generation, rather than young exclusively when preparing for update refs

[William Kemper]

Without this code, global collections will not function correctly.

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Commit messages:
 - Use active generation, rather than young exclusively when preparing for update refs

Changes: https://git.openjdk.java.net/shenandoah/pull/113/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=113&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/113.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/113/head:pull/113

PR: https://git.openjdk.java.net/shenandoah/pull/113