[dnsdist] expected file descriptor usage

[Christoph]

>> What will happen when it hits the limit
>> of 16k?
> 
> We might refuse new TCP connections, or if we fail to properly handle a
> case you might see dnsdist restarting after an unhandled exception.

Now we got to experience what happens when dnsdist reaches the 16k limit
in case anyone else wants to know (confirm or disagree)

- dnsdist does not restart (but that would make sense and would result
in "auto-recovery")
- backend resolver (running on the same OS) is up/down flapping
according to dnsdist
- dumpStats() reports a fd-usage of "0"


[...]
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'up'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'up'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'up'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'up'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
dnsdist[20176]: Problem sending carbon data: Too many open files
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'up'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'up'
dnsdist[20176]: Marking downstream 127.0.0.1:53 as 'down'
[...]

We restarted it manually.