calendar-server: Use the actual recurrence ID of events

calendar-server: Use the actual recurrence ID of events

Instead of querying the client for a list of objects and using
e_cal_recur_generate_instances() to get occurrences for each of
them, we can use e_cal_client_generate_instances_sync() which
combines the functionality of both functions. This doesn't only
save us some lines of code (yay!), but also gives us access to
the real recurrence ID of an event, so we can get rid of the hack
of faking one.

https://bugzilla.gnome.org/show_bug.cgi?id=748226