Improve performance of retrieving event and series lists

The following requests are quite slow when asking for a large number of objects:

• GET /admin-ng/events

• GET /api/events

• GET /admin-ng/series

• GET /api/series

An analysis revealed that most of the time is spent creating JAXB Unmarshaller objects for each single item.

The goal of this task is to improve the performance of the requests mentioned above.