[URGENT] Need review for libkcddb 6 before feature freze next week

[Albert Astals Cid]

https://phabricator.kde.org/D27973

http://www.freedb.org/ is shutting down at the end of March 2020 so this basically rips off all the freedb code and just makes libkcddb a musicbrainz frontend.

Ideally we'd kill libkcddb but there's a few apps still using it, so let's do this maintaince bump.

Can anyone spend some minutes reviewing?

Cheers,
  Albert