Split clone() to clone() and pre_next_op()

[Anssi Kääriäinen]

QuerySet and Query clone() methods are doing more than just cloning, they are also doing set up for next operation, changing the class of the clone if requested, and even throwing away some attribute values.

To make the code a bit more readable the clone() method should be splitted to clone() part and pre_next_op() part. This also allows creating inplace querysets that work precisely like full-clone querysets. A queryset is called inplace if it doesn't do cloning at all between operations. This again allows speeding up certain internal operations like model.save() and prefetch_related(), and also allows users to optimise heavy-to-create querysets by avoiding clone() overhead.

Patch available from ​https://github.com/akaariai/django/tree/splitted_clone. The inplace queryset used for Model.save_base() speeds up model.save() by about 20% for a simple m = Model.objects.get(); m.save() benchmark. There are other places where similar optimisations could be used. Prefetch speedup is tracked in ticket #20577.