Datasources might not be added / removed correctly from Cesium and Leaflet maps in some rare cases

[na9da]

After Cesiums' switch to real Promise api from when.js, the DataSourceCollection.add() api is fully asynchronous all the time. Previously, when passing a concrete DataSource as argument, the add() used to happen instantly without any delay. But after the switch, add() is always deferred.

This has some consequence to our code for adding datasources to cesium/leaflet maps. Our implementation (incorrectly) depended on this "instant" add effect to work correctly.

• The subsequent call to raiseToTop() might silently delete a datasource (because calling raiseToTop on an item that does not exist has an undefined behavior)
• Datasources might get addded multiple times to the DataSourceCollection

This is rare and difficult to repro by adding items from catalog, but might happen when programatically adding data sources.