calendar-server: Give each event an unambiguous ID

Each event returned by GetEvents includes the (currently unused)
UID, which is not always enough to unambiguously identify an event
(different calendar sources, recurring events, ...).
As we will start using the property to record events that have been
dismissed and should be persistently hidden from the calendar, change
it to a truly unique ID.

https://bugzilla.gnome.org/show_bug.cgi?id=744927