Delay rendering of models in migration operations when possible

[Markus Holtermann]

As it turns out, when we delay the rendering of related models up to the point where it's necessary (just before RunPython, afaict), we can cut down migration time to a quarter and maybe more: ​https://github.com/MarkusH/django/commit/6b8280e0582d778944585e966404252c4994185e#commitcomment-19719252

PR: ​https://github.com/django/django/pull/7589

Given that 3rd party migration operations may need to enforce model rendering like we do for RunPython, we probably need to tread this as a backwards incompatible change, I suppose.

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        if from_state.is_delayed and 'apps' in from_state.__dict__:
            del from_state.__dict__['apps']
        # ...