musicbrainz.lua: Use get_releaseid even if the album title is known

This renames `try_release` to `get_releaseid` and uses it to find the
MusicBrainz Identifier. That MBID is later used in the query passed to
try_query.

Not using the ASIN returned from the search server in the get_releaseid
call means one additional request is performed in the case that both the
artist and the album name are already known.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
musicbrainz.lua: Support coverartarchive.org

If the Cover Art Archive has artwork for this album, use the 500x500px
version of it.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
musicbrainz.lua: Use MBIDs from files, if they exist

If the MBID of the album is already in the `meta` table, use it to query
the MusicBrainz web service, instead of doing extra requests to the
search server.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>