The current implementation for some of the database handlers, where the
same method is used to retrieve either a single entity (when an `id` is
passed), a list of entities, or `None`, makes the typing and overall
design more complex.
This change simplifies database handlers, by having two separate methods
where appropiate:
* A method that receives an `id`, and returns either an entity, or `None`.
* A method that optionally receives filters, and returns (depending on
the current handler implementation) a list of entities, or a `Select`
object that allows chaining more SQLAlchemy operations.
