Cleanup the dns_master_dumptostreamasync task

Right now, we pass a "task" to the dns_master_dumptostreamasync() function, we should rather pass zone->loop or main loop (in case of cache) that will be used to launch the offloaded work. By doing this, we can cleanup the whole task shebang from masterdump. This is continuation of the dns_masterdump API refactoring that has been already started.