relay: Bump MAX_DETECTABLE_CPUS

I just stumble upon this interesting #define in tor that caps the number of CPU to 16. I was able to trace this back to commit 73d93c03 (September 2010) which doesn't indicate much why that value was used.

Nowadays, it is not even uncommon to see servers with 128 CPUs so I will propose that we raise this value to at least that so we can take advantage of more CPUs for our threadpool handling onionskins which is what is causing a big problem on our network at the moment.

An operator can always use NumCPUs to control this in case tor takes too much.