- added workaround for a backend leak that causes the "contacts" calendar
        backend to hold open an EBook for the local contacts (filed upstream
        at: http://bugzilla.ximian.com/show_bug.cgi?id=68533 ); this was
        causing e-d-s to never lose its last addressbook, and hence never quit.
        We workaround this by detecting this condition and exiting when it
        occurs, fixing bug #134851 and #134849.