descendants can be forced to reload

The methods #descendants, #parents, #children, #ancestors do not offer a forced reload.
They do not deliver the real state of the active records but the one last loaded from database or modified directly.
Since in a polymorph dag most modifications are done indirectly this is not of any use.
The tests show that the current implementation of these essential dag methods are errornous and buggy and unsatitsfying the expectations of users and model. The methods results' can not be trusted and depend on the execution of a random number of other methods.
In one word, the whole thing was a shit until this great commit.