CRM-14670: include target contact counter for mass mailings + fix exclude activity type

This commit, which fixed performance issues, had the unintended consequence of fixing a bug with the activity filters (Contact record -> activity tab).

In 4.4, if you exclude an activity type from the filters, it would cause an SQL query fail, and the tab would seem to never refresh (see on d44.demo.civicrm.org -> got to a contact -> activity -> exclude any activity type).