Change the number of IOCPs used on Windows to 2 on larger machines

[kouvel]

• There are some scaling issues on Windows with 1 IOCP on larger machines, and some fixes were made to scale better with 2+ IOCPs. So far, the data doesn't show better scaling with more than 2 IOCPs even on very large machines.
• Determine when to switch to 2 IOCPs by default

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