Asynchronous DNS / Yggdrasil server check

[cofob]

Now it takes a long time to check the DNS servers, and you cannot add a new one until the rest have not been checked. It is better to move the checks into separate threads so that the servers are checked faster, and you do not have to wait for the check of all servers to finish.

It would also be nice if the application immediately showed all added servers with the "pinging" status, and they could be selected.