Asset cooker multithreading

[Xymanek]

The cooker has a "multithreading" (it actually spawns worker processes) option that is meant to speed up the cooking process. It is currently disabled (-singlethread) for 2 reasons:

• The cooking output is printed directly (MT just prints Sending Job [SomePackage] to child x) - it was much simpler to investigate how the cooker works
• Something somewhere went sideways when MT was enabled, but I can't recall what exactly and it might not even be relevant anymore with so many changes to the cooking process

It will be nice if it we could get the MT to work, especially for big mods like lwtoc, Resistance Firearms, etc