Make Far multithreaded

[birdie-github]

Description of the new feature or improvement

I've noticed that certain operations, e.g. Ctrl + Q or Directory -> F3, on modern computers with SSDs are not as fast as they could be because Far processes all the files and directories consecutively. This also affects Far when it begins to copy or move large directories and it needs to process all the files therein. And File Search too.

This can be replicated on an HDD as well when the OS has cached all the file/directories metadata.

Proposed technical implementation details (optional)

Please consider making Far multithreaded.