- cache invalidation does not work across plugins, which the current code
  assumes to work. To fix this we will either need a global object which takes
  care of all SQL queries or use a RAMCache directly. The latter can be done
  via plone.memoize (see feedmixer).

- use a CMF testcase for testing instead of PloneTestCase.

- hook in PAS user deletion event and use that to invalidate caches.

- extend the property provider tests to also test non-string properties.

