0.7-dev Support fetching of multiple small boundingboxes in FeatureDownloader

[gidema]

As the data quality in OSM improves, differences between OSM and the open data sources are more and more scattered in small bounding boxes. This creates the need to be able to download features from multiple small bounding boxes in one request.
Current WFS servers don't support this in one request, so we have to create a FetchTask per bounding box in the FeatureDownloader class.