Use entry points to make scripts


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13407 e27351fd-9f3e-4f54-a53b-843176b1656c
Implement background automatic purge of missing users (not enabled by default). Required some api clean-ups
and support for scheduling without a valid directory record.


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13408 e27351fd-9f3e-4f54-a53b-843176b1656c