- Postpone setting an ID for a Movie until it's added to the database.
- Use larger chunks to count the lines of an input file.
- Skip creating temporary objects for the score-map.
A `RatingRow`'s `id` and `updated` are never used, but creating them
a million+ times is quite expensive, so initializing them with `None`
saves a lot of time.
`dataclasses`' `fields` function is also quite expensive; loading the
fields from a row directly saves a lot of CPU cycles.
We want to limit the number of movies, not ratings. By limiting the
number of movie IDs before loading all ratings with those movie IDs
we can achieve this.
