Fix #2310. Play by the 8tracks rules

because we cant to predict the real download links without the server, we must play by the 8tracks rules:
* max 3 skip in hour. but playlist contains min 8 tracks, so 3 isn't enough
* continue to the next song after the current track done. but we don't know the exact length of track

so, some is better than nothing, and currently the exiting plugin is useless.
I calculate the average track length from playlist length and track count, then sleep after download failed until that new request be considered as legitimate.

ugly, takes a long time, but works! 
Ewww, `++a` doesn't do what you think it does. It's the same as `a` or `+a` or `+++++a`, it just does nothing in this case. There is no increment operator in Python, you have to use `a += 1`. Thus this loop may be infinite because you never increment `download_tries`