[1.5.x] Fixed #21102 -- Cannot pickle/unpickle a QuerySet that prefetches related objects more than once (Pickling Idempotency).

Fixed the bug that a QuerySet that prefetches related objects cannot be pickled and unpickled more than once (The second pickling attempt raises an exception).

Added a new test for the queryset pickling idempotency.

It was introduced by the patch bac187c0d8e829fb3ca2ca82965eabbcbcb6ddd5.