public synchronized Collection<String> getAllHosts() {
        return Collections.unmodifiableCollection(new ArrayList<>(host2ApplicationId.keySet()));
    }
A host registry with a mapping between hosts (hostname as a String) and some type T
TODO: Maybe we should have a Host type, but using String for now.

@author Ulf Lilleengen
configserver/src/main/java/com/yahoo/vespa/config/server/host/HostRegistry.java