Use all available cores

[simonschmidt]

json2csv uses a lot of CPU, but only one core. json.Unmarshal seems to be taking up most of the time so I made a forked version that launches a whole bunch of them in parallel

A 207Mb json file with 241804 lines takes 15s to convert to csv with the single-threaded version, using all cores (2 real + 2 hyperthreading) it takes 7s. However using just one core on the multi-threaded version takes 17s It might be worth to keep the single-threaded version in the code and use that when running on one core.

parallel version