[Enhancement]: Limit number of concurrent track downloads

[wjbuys]

Describe the Feature/Enhancement

Configure a sane default limit for the maximum number of audio tracks that can be downloaded for local listening at the same time. Currently, it will try to simultaneously download all files.

Why would this be helpful?

From my testing, for similarly sized audiobooks that differ only in the number of audio tracks:

• a book with < 10 tracks downloads in seconds
• a book with > 100 tracks downloads in minutes

I suspect that trying to juggle 100s of open download connections is overwhelming the iOS download manager, or my audiobookshelf server, or both.

Future Implementation (Screenshot)

This would probably make sense as a setting under (hamburger menu) -> Settings (gear) -> Data Settings.

Audiobookshelf App Version

iOS App - 0.9.77

Current Implementation (Screenshot)

I'm looking at an audiobook downloading right now, and it has a high double digit number in the download progress indicator which is ticking down ever so slowly. Viewing download progress shows dozens of tracks partially downloaded, and none of them making much progress.

[wjbuys]

Hm, actually, I think this might be a bug; looking at the code for the download scheduler, it looks like the intention was to limit to 5 concurrent downloads:

https://github.com/advplyr/audiobookshelf-app/blob/master/ios/App/App/plugins/AbsDownloader.swift#L22C15-L22C43

However, I think that we're running into the same issue as ReactiveX/RxSwift#2538 (i.e. the tasks being submitted to the queue are themselves async and complete immediately), but I might be misreading things here. Either way, this seems like a bug rather than an enhancement then.