NonMoving: Don't do major GC if one is already running
Previously we would perform a preparatory moving collection, resulting in many things being added to the mark queue. When we finished with this we would realize in nonmovingCollect that there was already a collection running, in which case we would simply not run the nonmoving collector. However, it was very easy to end up in a "treadmilling" situation: all subsequent GC following the first failed major GC would be scheduled as major GCs. Consequently we would continuously feed the concurrent collector with more mark queue entries and it would never finish. This patch aborts the major collection far earlier, meaning that we avoid adding nonmoving objects to the mark queue and allowing the concurrent collector to finish.
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