Fix loading events from a day if there are multiple instances with same uid + small changes

See invidividual commits.

Biggest problem of these is having recurring event with an exception and then modifying the main event time.
One occurrence in the main event was hidden by exception time which does not anymore apply after modification, so one day might include both occurrence from the main event and the exception. Loading this was failing due to badly created identifier.

As further development, the exception should probably be created as whole day or then adjusted together with main event when changing time.