Add change tracking for email address and username on CassandraUser
Support usernames potentially changing

- Use the change tracking in the CassandraUser class for usernames to determine what to do to the users_by_username table on Create/Update/Delete
Implement FindByEmailAsync

- Add another table and modify the user data in the users_by_email table on create/update/delete
- Use the change tracking for email in CassandraUser to determine what to do