Lookup existing events even when creating a new event

Summary:
If the client doesn't know about an already existing event, we will
otherwise overwrite whatever was there (because the uid and therefor
serverid will be the same). In case of outlooks dummy event
on acceptance that means resetting the attendee status to 0 (after it
was set to accepted via the MeetingResponse).

Differential Revision: https://git.kolab.org/D4457