Use the 'log' package (nearly?) everywhere
This isn't a race condition AFAICS
Fix panic after stopVideo during buffering

Check for the condition and make sure the condition shouldn't happen in
the first place.
go fmt
YouTube: check for non-200 HTTP status codes
Use mpv again! Requires MPV 0.6.0

Additionally, this fixes some API changes in MPV.

This switch was made because MPV is much more stable (although the API
is still in flux). The downside is that seeking long streams is still
quite slow (about 15s on the Raspberry Pi for an one-hour stream, see
https://github.com/mpv-player/mpv/issues/1183).
Some documentation