Reduce log spam when upstream interface is offline

[rbubley]

This was the suggested fix to a problem observed in the OpenWrt implementation of stubby (usually embedded in a router), where if the upstream interface went offline, the logs would rapidly be filled with "Could not schedule query". This fix was suggested by Andre Heider via @dhewg (See openwrt/packages#13726)

[dhewg]

Yeah, and I'm still using this patch.

But in this state it's probably not suitable for upstream. A nicer solution might be to only skip that line if the prior error was the same.

[GektorUA]

Stubby 0.4.3 + mwan3, now on OpenWrt is no messages at all about "Could not schedule query..." when there is all links offline. And when one of links goes online, stubby hangs, needs to do manual restart of stubby to make DNS works.