Manage DNS state better when "All nameservers have failed"

We should downgrade this warning when it only happens for a short period of time (or a small number of requests), or when it happens in response to a malformed request.

This warning is causing operators to make sub-optimal DNS server choices: for example, avoiding using a local cache in favour of remote resolvers.

Sometimes changing the local resolver makes a difference: https://trac.torproject.org/projects/tor/ticket/1936#comment:12

Sometimes it happens in response to malformed requests: https://trac.torproject.org/projects/tor/ticket/11600#comment:6

Sometimes it's harmless: https://trac.torproject.org/projects/tor/ticket/11600#comment:7 Because it's followed by:

[notice] eventdns: Nameserver <ISP-resolver2>:53 is back up