dnsdist: Fix an outstanding counter race when reusing states

[rgacogne]

Short description

I finally managed to reliably reproduce the "negative outstanding counter" issue by setting setMaxUDPOutstanding() to a very low value and overloading the backend, causing dnsdist to do a lot of "active state reuses".
This PR tries harder to detect whether another thread has been reusing the state, making sure that we don't decrement the outstanding counter twice.

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)