Big refactoring / performance improvements

[timrae]

I've put up PR #43 over the weekend introducing numerous performance improvements.

Taken together with my last PR #41, I would expect around an order of magnitude speed improvement for the case where you are regularly calling the script and there are just small incremental changes since the last sync. However the script will periodically go through previous unmatched Spotify tracks to see if they have been made available on Tidal since the last sync. In this case the speed improvement may be less dramatic if there are a lot of tracks to go through.