Implement abort for FeatureResponse

So we now have this great abort structure, which will detect if a client has closed the connection and GeoServer can stop using the resources that are being spent on that client. This is done quite well in the SVGMapResponse, but really no where else. It does not matter so much for something like GetCaps, or DescribeFeatureType, as they are not super resource heavy. But we really should have it in place for FeatureResponse. On the geoserver side this will involve adding an abort method to FeatureResponseDelegate and having FeatureResponse call it when its abort method is called. Then for gml we need the geotools gml producer to implement an abort method as well, so it stops loading.