Fixed #11295: If ModelAdmin.queryset returns a filtered QS don't require a 2nd count call

Original patch rewritten, added tests and get_filters_params method for ChangeList class.
Thanks Alex for the report.
URL-encoded URLs in FlatPage.get_absolute_url.
Fixed #18862 -- Honored script prefix in FlatPage.get_absolute_url.
Merge pull request #820 from viciu/11295

Fixed #11295: If ModelAdmin.queryset returns a filtered QS don't require a 2nd count call
Fixed #19810 -- MemcachedCache now uses pickle.HIGHEST_PROTOCOL
Fixed minor rst formatting glitch.
Merge pull request #819 from erikr/master

Fixed #16302 -- Ensured contrib.comments is IPv6 capable.