The performance of DMD hasn't been particularly great - hence why
there's been the need to introduce a progress bar for most of the
operations - and it turns out it's to do with SQLite's transactions[0].

As we're not wrapping any of our updates in a transaction, SQLite is
producing one for us, which is slower than us managing it ourselves, as
we know how many `INSERT`s we'll be doing at a given time.

This speeds up considerably, for instance the `integration_test` phase
currently takes ~90 seconds instead of ~240 seconds, and results in no
longer even seeing the progress bar on `import`s.

Closes #89.

[0]: https://stackoverflow.com/questions/3852068/sqlite-insert-very-slow