Simplify entity equality code and enfore db/metadata split
Use a nuget package to remove boilerplate code that needs careful
update when adding/removing fields. Add tests to enforce that all
fields are allocated to 'UseMetadataFrom' or 'UseDbFieldsFrom' to make
metadata refresh more foolproof.
Fix NRE when tracks are merged because artist wasn't set.
Fix NRE when tracks are merged and the merge target wasn't yet in the database.
ta264