Use RPC endpoints to load map markers

[BurntimeX]

MarkerLoader always queries the map markers using the method called getMapMarkers in a defined DBO action. Instead, it should be possible to specify an RPC endpoint for querying the data.

In addition, the following issues should be solved:

• getMapMarkers() fails when map crosses the international date line
• getMapMarkers() excludes valid records at equator/prime meridian (undefined values should be saved as NULL and not as 0)
• The validation and calculation of the coordinates is always identical and should be abstracted as much as possible.