Instead of fetching DatasetModel one by one, do a bulk fetch into a dict
to save roundtrips to the database.

Prior to this commit, we already only create DatasetModel rows inside
the manager. This also changes how DatasetAliasModel to only be created
inside create_dataset_aliases, and only associated them to DatasetEvent
in register_dataset_change.

All the dataset manager functions are also changed to only accept
public-facing dataset classes, instead of ORM models.

The register_dataset_change function now takes an additional keyword
argument 'aliases' that is a list of dataset aliases associated to the
DatasetEvent to be created.

Prior to this commit, we already only create DatasetModel rows inside
the manager. This also changes how DatasetAliasModel to only be created
inside create_dataset_aliases, and only associated them to DatasetEvent
in register_dataset_change.

All the dataset manager functions are also changed to only accept
public-facing dataset classes, instead of ORM models.

The register_dataset_change function now takes an additional keyword
argument 'aliases' that is a list of dataset aliases associated to the
DatasetEvent to be created.