ycsb: fix loader to CPU allocation to be NUMA aware
ycsb: fix loader to CPU allocation to be NUMA aware

a bit of a hack, since we want to use all available cores to load the
tree (to speed up benchmarking time), but we also want to distribute the
memory as if we had only the number of threads running in the benchmark
also loading the tree.

nathanielherman/silo,nathanielherman/silo,stephentu/silo,stephentu/silo,nathanielherman/silo,nathanielherman/silo,stephentu/silo,stephentu/silo