Rework the Aria2c Downloader

- Downloads are now multithreaded directly in the downloader.
- Now reuses connections instead of having to close and reopen connections for every single download.
- Progress updates are now yielded back to the caller instead of drilling down a progress callable.
- Instead of parsing download progress information in a very hacky way from the stdout stream, use aria2's RPC interface.
- Added a new utility get_free_port which is needed to choose aria2's RPC port as I do not want to use the default port in case the user is already using this port for another tool or reason. Also, to try mitigate port scanning attacks that target aria2 RPC ports.
- The config entry `aria2c.max_concurrent_downloads` is now actually used by aria2c when downloading.
- The `--max-concurrent-downloads` option and config value now defaults to `min(32,(cpu_count+4))` (usually around 16 for above average systems) instead of 5.
- Automated pproxy proxy rerouter is made via subprocess instead of trying to re-do what the pproxy entry point does for us, less code, less trouble, and was ultimately easier to implement.

CONFIG.md

@@ -11,13 +11,12 @@ which does not keep comments.
 ## aria2c (dict)
 
 - `max_concurrent_downloads`
-  Maximum number of parallel downloads. Default: `5`  
-  Note: Currently unused as downloads are multi-threaded by Devine rather than Aria2c.
-  Devine internally has a constant set value of 16 for it's parallel downloads.
+  Maximum number of parallel downloads. Default: `min(32,(cpu_count+4))`  
+  Note: Overrides the `max_workers` parameter of the aria2(c) downloader function.
 - `max_connection_per_server`
   Maximum number of connections to one server for each download. Default: `1`
 - `split`
-  Split a file into N chunks and download each chunk on it's own connection. Default: `5`
+  Split a file into N chunks and download each chunk on its own connection. Default: `5`
 - `file_allocation`
   Specify file allocation method. Default: `"prealloc"`