eggjs/benchmark does not make use of all the cores available

* **Node Version**: all
* **Platform**: Intel Xeon

The number of workers is hard-coded in `dispatch.js` of all cases e.g. :
https://github.com/eggjs/benchmark/blob/master/simple/dispatch.js#L5-L8

And the number of threads used to run the benchmark is also hard-coded, e.g.:
https://github.com/eggjs/benchmark/blob/master/simple/run.sh#L51-L55

We should also make it possible to fire requests from another machine instead of the one serving them.