Also do garbage collection to remove references from non-persistent objects

_objects member BaseTemplateItem class is a PersistentMapping object, which
contains a _dict_ of persistent objects. In some cases, this dict remained in
memory, preventing pickle cache to remove objects with temporary classes.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@32811 20353a03-c40f-0410-a6d1-a30d3c3de9de