feat(dl): Add *new* --workers to set download threads/workers

The previously named --workers which is now --downloads specified how many tracks to download, not how many threads/workers are used per-download.

It defaults to nothing, which each downloader then has their own defaults. All current downloaders though currently default to `min(32, (os.cpu_count() or 1) + 4)`, which is also the default for `ThreadPoolExecutor` in general.

This also brings a side effect of changing DASH and HLS's forced max_workers of 16 to now a more appropriate default but more importantly actually configurable. You can set a default in your config under `dl.workers`.

CONFIG.md

@@ -153,6 +153,13 @@ For example to set the default primary language to download to German,
 lang: de
 ```
 
+to set how many tracks to download concurrently to 4 and download threads to 16,
+
+```yaml
+downloads: 4
+workers: 16
+```
+
 to set `--bitrate=CVBR` for the AMZN service,
 
 ```yaml