Optimal -N for core count

[mpickering]

@omnibs

I also tried the max heap size I saw in ThreadScope (128MB), which means we ran ~1 GC per second, but memory usage climbed a bunch, from <200MB to >1GB.

-A sets the size of the nursery for each capability. So if you set -A128M -N8 then the baseline for your application will be at least 1GB (128 * 8).