Reorganised the internals of the Where node a bit to fix some copying problems.

We no longer store any reference to Django field instances or models in the
Where node. This should improve cloning speed, fix some pickling difficulties,
reduce memory usage and remove some infinite loop possibilities in odd cases.
Slightly backwards incompatible if you're writing custom filters. See the
BackwardsIncompatibleChanges wiki page for details.

Fixed #7128, #7204, #7506.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7773 bcc190cf-cafb-0310-a4f2-bffc1f526a37