Implement configurable number of cpus for stress testing

[lzchen]

          for a follow up PR - make the count configurable instead of defaulting to num_cpus. We can use it to measure how good we scale by trying different thread count from 1, 2, ...till num_cpus.

Originally posted by @cijothomas in #1045 (comment)

[cijothomas]

@ThomsonTan Do you want to help fix this, since you are already in the stress testing area!