dnsdist: DOH - make query channel receiving part blocking

[chbruyand]

Short description

When not in single acceptor mode, the DOH thread processing incoming queries after they have been parsed is using 100% as the incoming channel is not blocking. This PR makes the receiving part to block.

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)

[rgacogne]

Nice catch!

[rgacogne]

I introduced that bug in the channel PR, so after 1.8.0, fortunately..

[rgacogne]

I think it's time to replace the booleans by enum class objects so it's clearer what they mean, I'll do that in a follow-up PR.