bind memoize cache to object instead of function to limit cache to object lifetime
bind memoize cache to object instead of function to limit cache to object lifetime

fixes memory leak when configurations are reloaded inside mapproxy (closes #84)

mapproxy/mapproxy,olt/mapproxy,vrsource/mapproxy,faegi/mapproxy,procrastinatio/mapproxy,faegi/mapproxy,kaiCu/mapproxy,camptocamp/mapproxy,geoadmin/mapproxy,olt/mapproxy,camptocamp/mapproxy,drnextgis/mapproxy,procrastinatio/mapproxy,kaiCu/mapproxy,vrsource/mapproxy,mapproxy/mapproxy,Anderson0026/mapproxy,geoadmin/mapproxy,drnextgis/mapproxy,Anderson0026/mapproxy