RFR: Remove young generation access to old generation mark queues earlier

[William Kemper]

Prior to this change, we removed access to the old generation mark queues if (and only if) the old generation cycle completed. However, in some cases, the old generation cycle may be interrupted _after_ it has finished marking but _before_ it has finished preparing for mixed collections. This error may cause young generation cycles to enqueue old objects into the old generation mark queues that may become invalid as mixed collections execute. With this change, we remove access to old generation mark queues as soon as old generation marking is complete.

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Commit messages:
 - Remove young generation access to old generation mark queues when old marking is complete

Changes: https://git.openjdk.java.net/shenandoah/pull/120/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=120&range=00
  Stats: 8 lines in 2 files changed: 5 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/120.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/120/head:pull/120

PR: https://git.openjdk.java.net/shenandoah/pull/120