* Add the `csv` crate as a dependency
* Use the `Row` struct as intermediate object
* Turn the `parse_value` function into the `convert_value` function that
cannot fail
* Replace the lazy `once_cell` by a maps handle type
* Use Rocket's managed state to manage a handle
* Ensure that the handlers have access to it
* Pass another handle to the maps updater loop
* Try to keep the lock as short as possible
Still, long downloads block the lock. Add a FIXME to refactor this
so the lock is only taken when updating the maps fields.
Paul van Tilburg