Fix #348, #474, #557, #740, #928: improved deduplication logic to prevent skipped uploads

Improved deduplication logic to prevent skipped files during import

Closes #348 (closed), #474 (closed), #557 (closed), #740 (closed), #928 (closed)

Some of these bugs are duplicates, but the core issues where:

• Considering a TRACK_ID (Track.mbid in Funkwhale) was unique, for instance leading to orphaned/skipped uploads depending on the processing order. Solved by doing the unicity check with the ALBUM_ID and TRACK_ID combination
• Not including the disc number when trying to deduplicate tracks, leading to skipped uploads when a track with the same title was present at the same position but in a separate disc

I'm still testing this locally with real-world data, hence the wip.