FR: Multithreaded library synchronization

[romner-set]

Description
At the moment, php artisan koel:sync uses a simple foreach loop to iterate through all songs and add them to the database, occasionally stopping to update the progress bar. This can take quite a long time on large libraries no matter the hardware involved (my 50k FLAC song library takes upwards of 40 minutes to scan for the first time with a 7950X3D, even when stored on a Samsung 980 PRO with memory_limit set to 8192M). I think this could be improved if the work was split up into multiple threads that run simultaneously.

Making 1 thread per song would probably be excessive for large libraries, and since the CPU usage while scanning is very low I don't think 1 thread per CPU core would make much sense either, though that can only really be verified by testing.

Example
N/A

[phanan]

PR welcomed.