dnsdist: Implement ref counting for the DOHUnit object

[rgacogne]

Short description

It turns out that, at least when testing with ASAN enabled, we sometimes trigger use-after-free detection because we get the response from the backend, send it to the client then delete the
object before the send() call to the backend even returned.

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)