Adds ExistsExpression (true if an attribute has a value) and BooleanExpression (true if the associated directory attribute has a specific value)


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13867 e27351fd-9f3e-4f54-a53b-843176b1656c
Added support for fetching delegate assignments from the directory


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13868 e27351fd-9f3e-4f54-a53b-843176b1656c