IterativeResolver not resilient to root server failure #144

If a root server is down, and you can only use IPv4 or IPv6, IterativeResolver seems to only query 1 root server. So if that server happens to be down, resolution fails.
And it takes 10 seconds (by default) to fail because it takes 5s to fail with UDP, and it then tries TCP which also takes 5s to fail. It would be nice have the option to disable fallback to TCP in this case.
A choice of 1 random IPv4 root server and 1 random IPv6 root server does not seem to be a good strategy in case of root server failure.