8373819: GenShen: Requested generation may be null

[earthling-amzn]

This PR attempts to simplify the generational control thread by decoupling it somewhat from the heap/gc cancellation mechanism. This is meant to prevent the control thread from seeing inconsistencies between shHeap::_cancelled_gc and shGenControlThread::_requested_gc_cause.

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Progress

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Issue

• JDK-8373819: GenShen: Requested generation may be null (Bug - P4)

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[openjdk]

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[mlbridge]

Webrevs

• 00: Full (8b3acc85)