New CachedDataset and CachedDatasetElement models in cache

New training worker will need to know the set of an element without doing an actual request if possible.

We need two models:

from peewee import SQL
class CachedDataset(Model):
    id = CharField(primary_key=True)
    name = CharField()
    state = CharField(constraints=[SQL("DEFAULT 'open'")])
    sets = TextField()


class CachedDatasetElement(Model):
    id = CharField(primary_key=True)
    element = ForeignKeyField(column_name="element_id", field="id", model=CachedElement)
    dataset = ForeignKeyField(column_name="dataset_id", field="id", model=CachedDataset)
    set_name = CharField()

    class Meta:
        table_name = "dataset_element"

We'll bump the Cache version as well.

Only introduce the models, then we'll work on the helpers.