Cleanup: add additional annotations for often used conversions

[nbartels]

At the moment we use a JsonMappingCompleted method to do some stuff after the mapping. Often used is the date conversion, because the mapper don't knows about this in the first place.
We let the mapper write the transferred date as String in a raw field and let the post-mapping method convert this String into the java Date object afterwards.

This results in many code duplications and lot of raw fields, that are only added to the object for this very special use case. I suggest to add a Date conversion from JSON to Date. The other way around is already implemented and we can convert a Date to JSON.

With this change we stay backwards compatible, because we have the possibility to save the provided string containing a date as String and to a more complex conversion afterwards. But for the common case we don't need two fields and no post mapping methods 😄