resolved: Recover from slow DNS responses

When DNS is unreliable temporarily, the current implementation will
never improve resend behavior again and switch DNS servers only late
(current maximum timeout is 5 seconds).

We can improve this by biasing the resend_timeout back to the current
RTT when a successful response was received. Next time, a timeout is hit
on this server, it will switch to the next server faster.

Fixes: #5953