_process_asin_relation(): reduce indentation level
_walk(): reduce indentation level
_fill() -> _fill_from_relationships()
_coverart_downloaded(): better handling of not enough data case

At least log a message in debug mode in this case.
Btw, i'm not sure why this condition was added, even if it makes sense.
Drop useless coverartobj argument, cleanup
Use types intersection to select caa images to download, clarify code

- lowercase types from json, to match lowercased types from options
- fix up logic concerning caa_front_found
- add few comments and debug info
Subclass CoverArtImage and add support_types property

- CaaCoverArtImage is only used for CAA images
- drop host comparaison (vs 'archive.org'), use a much cleaner "flag"
- all current providers but CAA don't support multiple types, so we defaults to a "front" type
- once we have one image from one of those providers we are done
- we don't want to remove all images from CAA having front type
Use an explicit is_front property for CAA front flag indicator