Memory leak in WMS response handling

DefaultMapContext attaches listeners to the feature source, these listeners are referenced by FeatureListenerManager and are never cleaned up, meaning each request builds up a new featuresource/defaultmapcontext couple in featureListenerManager. The leak is usually small, but can become sizeable if the styles used by the map context are big.

We should clean up after ourselves when the response is done (the cleanup code there, but for some strange reason it has been commented out when merging the WCS branch one year ago), but at the same time FeatureListenerManager should be smarter and use weak references to avoid this kind of issues