dnsdist: Skip signal-unsafe logging when we are about to exit, with TSAN

dnsdist: Skip signal-unsafe logging when we are about to exit, with TSAN

TSAN is rightfully unhappy about this:
```
WARNING: ThreadSanitizer: signal-unsafe call inside of a signal
```

This is not a real problem for us, as the worst case is that
we crash trying to exit, but let's try to avoid the warnings
in our tests.