Rework how we implement primary keys in models

~Waiting on https://github.com/django/django/pull/18365~, should re-work unique fields afterwards, https://github.com/surister/cratedb-django/commit/e34bfafc78a4cbbc31778366347f6a7367602526 and https://github.com/surister/cratedb-django/commit/541ddb25d3155189644d144694f8e69e46ac62e3,  it simplifies how we do primary keys in models