Add serialVersionUID to datastore entities #22
Description
Now that schema checking has been weakened, the need for some kind of versioning for entity definitions is more useful.
An (overridable) serialVersionUID field should be generated from a hash of the properties on the entity. A schema value which has a different value for the serialVersionUID of the entity should raise an error when fetching the value from the datastore.
By overriding and adjusting the value of the version UID, changes to entities can be easily separated into
- changes which break the existing kind definition.
- changes which do not break the existing kind definition.
A warning should be emitted for entities which do not override the serialVersionUID field.