[Audio] Save current track progress on async cog_unload

[Zephyrkul]

Type of feature request

Other

Description of the feature you're suggesting

As discussed in #advanced-coding in the Red server.

With discord.py 2.0's new asynchronous cog_unload, it should now be possible for Audio to save current track progress in cog_unload for resuming when it starts up again. For reference, current behavior only resumes from the first track in the queue rather than the playing track.

Anything else?

No response