Multi-threading for pack/unpack

[amisevsk]

Describe the problem you're trying to solve The pack and unpack commands run through layers sequentially, one at a time. This means that if e.g. a modelkit has 3 layers, each 100MB in size, packing/unpacking will take roughly 3x as long as if it was unpacked in parallel.

Describe the solution you'd like Unpack layers in parallel.

Describe alternatives you've considered N/A

Additional context Add any other context or screenshots about the feature request here.