RFR 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[Zhengyu Gu]

> 
> 
> You need to use the comment button at the top! Not the one at the
> bottom. Because the bottom one doesn't work. :-)
> 
Thanks, Roman.

I made corresponding changes in CSR.

-Zhengyu


> Roman
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>> Anyway, could you please review CSR also?
>>
>> FYI:
>>   Webrev02 passed vmTestbase_vm_gc and submit tests.
>>
>> Thanks,
>>
>> -Zhengyu
>>
>>
>>
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>>> Roman
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