* Change return value of `SyncChaptersWithSource.await()`
`updatedToAdd.subtract(reAdded).toList()` never worked as at this point `updatedToAdd` contained ids from db where `reAdded` had default one. Was the same case before the rewrite.
Removed `toDelete` from return value as it was not being used anywhere
* Add doc string
* Use HashSet
Co-authored-by: stevenyomi <95685115+stevenyomi@users.noreply.github.com>
Co-authored-by: stevenyomi <95685115+stevenyomi@users.noreply.github.com>
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