musicbrainz: change API to mbzero

mbzero is much lighter and things have to be implemented in beets:

- there is no helper such as get_release_by_id
- no checking, so wrong requests will raise an exception
- there is nothing to limit the requests, it needs to be implemented
- mocks are moved to the interface

Signed-off-by: Louis Rannou <[email protected]>

Author: Louis Rannou