Better metadata lifecycle support

[benoitg]

When eureka was originally designes, it was internaly modeled after the LOM Data model. This is extremely limiting for metadata lifecycle, and must be improved. Specifically, the following should be added:

• Record the date a resource was added
• Record the date a resource was last modified. Upon import, if the data can be determined from the metadata, this date is used. Ideally, the date should only be updated if there is an actual change in the metadata, not if somebody merely changed page in the metadata editor. Unfortunately this will require extensive work in the different object abstraction to notify the edit context that objects changed (and ideally which ones). It may however provide the opportunity for some nice performance improvement by allowing to skip full recursion of the processAdminUI() calls.
• Record the date a resource was approved.
• Record the date a resource was deleted.
• Keep the lifecycle information of deleted resource for good federation support (OAI-PMH in perticular). Once this is done, do not forget to skip such resources when checking duplicate OAI-Id on import. An imported resource with the id of a deleted one should silently replace it.