RFR: Clear young generation reference to old mark queues when starting global collect

[William Kemper]

If concurrent marking in old is preempted by a _global_ collection, the old mark is abandoned. The young generation should no longer hold a reference to the old mark queues.  Without this fix, collections in young generation may erroneously behave as though concurrent mark in old is still running.

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Commit messages:
 - Young generation should not hold reference to old mark queues after a global collect

Changes: https://git.openjdk.java.net/shenandoah/pull/99/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=99&range=00
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/99.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/99/head:pull/99

PR: https://git.openjdk.java.net/shenandoah/pull/99