rework the ExportPool class

[pinhead84]

The org.openestate.is24.restapi.utils.ExportPool class is implemented in a needlessly complicated fashion. For example it makes problems, if the an object id contains some special characters. As a workaround to that problem we've implemented a separate internal object ID within the pool. But we should find a better / cleaner / more straightforward implementation, that does not require a separate ID.

The reimplementation should make org.openestate.is24.restapi.utils.ExportPool easier to use and less vulnerable in certain border cases.