Currently, the lifetime of the mDNS service is tied to the lifetime of the StunAddrsRequestParent, which is not freed until the process is shutdown (see Bug 1569311). With a bit of care, we possibly could shutdown the mDNS service after an unregister_hostname call if no more hostnames are registered.

We might want to have an idle period prior to shutdown. A site like aliexpress ends up registering and unregistering hostnames during page to page navigation, and having some sort of timeout would reduce the number of times we start and stop the service.