some iterator states for nonresponsive domains can get into a
state where they waited for an empty list.
- Stop UDP to TCP failover after timeouts that causes the ping count
to be reset by the TCP time measurement (that exists for TLS),
because that causes the UDP part to not be measured as timeout.
git-svn-id: file:///svn/unbound/trunk@4912 be551aaa-1e26-0410-a405-d3ace91eadb9