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

[rgacogne]

Short description

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.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[omoerbeek]

It would be better to set a flag and the check for that flag, but this is OK for now.