replace child document

when replacing a mapped child with a new document, we get the cannotMoveByAssignment exception, which imo is a) the wrong message as its not about setting a tracked document to become a child somewhere else but what we should do is delete the existing child document and add the new one.

this happens at https://github.com/doctrine/phpcr-odm/blob/master/lib/Doctrine/ODM/PHPCR/UnitOfWork.php#L1912

for multiple children, i am less sure how it should work, if it is possible the same way:
https://github.com/doctrine/phpcr-odm/blob/master/lib/Doctrine/ODM/PHPCR/UnitOfWork.php#L935