DS-1990: Events should contain all identifiers, not only handles.

This adds a field identifiers to the event class that contains all identifiers of an DSpaceObject.

Currently the IdentifierService is called to create identifiers only when an Item is submitted. Nevertheless this PR uses the identifier field for every DSpaceObject as the IdentifierService supports to register identifiers for every DSpaceObject.

To achieve this this PR extends the IdentifierService, the DSpaceObject class and the initialization of Events.