Perf improvements with concurent hash calculation
=================================================

Using the source code dataset here are the new times:

`19:38:46.745` -> `19:41:56.652` = `00:03:09,907`

But this time I also had the good idea to close all my processes and to use
a tmp directory for writing.

With the same setup, the previous perf was:

`19:26:05.954` -> `19:29:20.805` = `00:03:14,851`

So not that big of an improvement but it seems that at the same time CPU usage
has decreased a bit. Maybe because less synchronisation calls were made ?