dnsdist: Stop reporting timeouts in `topSlow()`, add `topTimeouts()`

dnsdist: Stop reporting timeouts in `topSlow()`, add `topTimeouts()`

Until this commit `topSlow()` returned queries that timed out, which
is not very helpful. This was happening because timeouts are internally
recorded with a very high response time.
With this change, `topSlow()` now ignores queries that timed out, and
a new command is added to look into these: `topTimeouts()`.