WT-18721 fixes data-source statistic buckets at 4, because each open btree dhandle carries its own copy and a disagg ident has two of those. Connection statistics stay at the fixed 23 chosen in 2015, when machines with more than 24 CPUs were rare and WiredTiger could not see how many CPUs the process was allowed to run on.

At connection open, take the process's allowed CPU count. On Linux that is sched_getaffinity, which MongoDB already uses, and where that is not available keep a fixed fallback. Set the connection bucket count from that number. Set the data-source bucket count to min(8, cpu_count). The count does not need to be prime: the hot path indexes by session id, which is a dense slot number. Do not look up the current CPU on the statistics update path.