RFR 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[zgu at redhat.com]

Hi Thomas,

Could you please also review the CSR: https://bugs.openjdk.java.net/bro
wse/JDK-8214458

Thanks,

-Zhengyu

> Looks good otherwise. I do not need a re-review for above change. I
> will have this final change run through our testing one more time and
> will report back (tomorrow).
> 
> Thanks,
>   Thomas
> 
> 
>