MOB-2213: Add information about poi count to map CRUD API

@inez @nandy-andy @RafLeszczynski I'm fine with map_id querystring param, as it just introduces one additional property for the db filter, but city_id is currently not kept by the POI object in DB so it gives us performance questions again :) What is the exact purpose of adding city_id to the querystring for POI if maps are already crated per city_id?