Multithreaded song update

[phu54321]

Hello, I'm trying to host BMS files on a webdav server. Beatoraja takes too much time to read all songs, as all requests are done sequentially.

This also affects reading large BMS sets (like ultimate.zip containing 8000+ bms files) as SSD read speed aren't saturated. To mitigate this, "Update BMS Database" function should be re-implemented using thread pool technique.

I could create a PR for this. Thanks.