Remove quadratic behavior in ``xml.minidom`` node ID cache clearing.  In order
to do this without breaking existing users, we also add the *ownerDocument*
attribute to :mod:`xml.dom.minidom` elements and attributes created by directly
instantiating the ``Element`` or ``Attr`` class. Note that this way of creating
nodes is not supported; creator functions like
:py:meth:`xml.dom.Document.documentElement` should be used instead.
