RFR: Parallelize safepoint cleanup

[Kirk Pepperdine]

Hi Roman,

I also see opportunities for some wins with this enhancement.

>> 
>> 
>> 
>> - Then I gave SafepointSynchronize its own workers, and I now think this
>> is the best solution: indepdent of the GC and relatively isolated code-wise.

This seems like the best option to me as everyone would benefit irregardless of choice of collector. For example, serial is still by far the best option for smaller devices/heaps. Additionally, I’d be hestiant to place more load on the GC threads.

Kind regards,
Kirk