Add "threads" option / multi threaded compression to 7z + LZMA

[ngladitz]

#107 implements this for xz. It would be great if the same could be done for 7z.

[mirostauder]

+1 on this since p7zip on linux is stuck on 16.02 and no update in sight, libarchive is the only other option to make 7z archives on linux.

[tjaalton]

yes please, extracting a disk image at 15MB/s with one core occupied is just pain I'd have not thought to see in 2021

[tjaalton]

and I was talking about decompression.. if that's not doable then meh..

[Osyotr]

Bump. This is very much needed.

[moralground]

Still need this badly.