disambiguate recordings of remasters in the docserver

[alastair]

A change in musicbrainz policy means that remasters should have the same mbid. This means that we can have two recordings with different audio and the same mbid. We can't store them both in the docserver.

Options:

• Store just 1
• disambiguate with the release id (we could have a remaster on the same release though)
• disambiguate with track id (guaranteed unique, but isn't really exposed in musicbrainz or dunya)
• Use track id in the docserver without using recording id

[sertansenturk]

I will note all such recordings here: https://docs.google.com/a/upf.edu/spreadsheet/ccc?key=0Aswz5KkuLnI3dDRYZHVUNzl2dllHVlBSVDVNek5YcWc#gid=4

[alastair]

already reported in #181