New files:
* `abstract_sql_dao.dart`: DAO abstraction for SQL.
* `bulk_deletable.dart`: Interface for bulk database deletes.
* `bulk_insertable.dart`: Interface for bulk database inserts.
* `bulk_manager.dart`: Manager for bulk database inserts and deletes.
* `dao_hive_product.dart`: moved code from the previous `hive` version of `DaoProduct`, renamed here `DaoHiveProduct`
Impacted files:
* `dao_product.dart`: `sqflite` version of `DaoProduct`, that is from now on the only one to be used
* `local_database.dart`: added `sqflite` and its one table so far, also named `DaoProduct` in order to minimize the changes everywhere else (the old `DaoProduct` being renamed `DaoHiveProduct`); migrated the product data from `hive` to `sqflite`
* `Podfile.lock`: wtf
* `pubspec.lock`: wtf
* `data_importer/pubspec.yaml`: updated the `sqflite` dependency to the same as in `smooth_app`
* `smooth_app/pubspec.yaml`: added a dependency to `sqflite`
