Acquisition total comments

The problem is described here: https://github.com/plone/Products.CMFPlone/issues/324

In short, two problems:

- if ``View`` permission is acquired, then ``total_comments`` does not return the proper result (due to not using acquisition wrapped comments)
- upon catalog reindexes ``total_comments`` *again* doesn't do its job properly, the *funny* thing is that if you remove the ``@property`` decorator it does work (as the second commit shows).

Opinions?