8382089: G1: Refactor concurrent refinement state machine

[albertnetymk]

Streamline the content of `G1ConcurrentRefineThread::do_refinement so each step/state is clearly visible, which is a good entry point for reviewing this refactoring. The largest diff is for step5/6, which is extracted into helpers just like the previous steps.

For state management, added enter_state(State, Ticks) that atomically updates state and records timestamp.

Test: tier1-7

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• JDK-8382089: G1: Refactor concurrent refinement state machine (Enhancement - P4)

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[mlbridge]

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• 01: Full - Incremental (2705ba31)
• 00: Full (277272a9)

[tabata-d]

I believe this GC log, which displays the number of worker threads, is meaningful. Should it really be deleted?

[tabata-d]

I haven’t looked at this log, so I can’t say whether it’s necessary or not, but is deleting it appropriate?

[albertnetymk]

Restored both logs. I think "log_debug(gc, refine)("Yielded from card table sweeping ..." is not very useful, but can discuss/address that in a separate PR.