dyn_images: choose country explicit via url parameter

Allows better caching
dyn_images: finally limit extent to maximum EPSG3857 extent
dyn_images: join and eager_load tables used in filters

Fixes performance issues, but produces a SQLAlchemy warning:
AWarning: Column xxx on table yyy being replaced by Column zzz,
which has the same key. Consider use_labels for select() statements.