dnsdist: Add `setTCPUseSinglePipe()` to use a single TCP waiting queue

[rgacogne]

Short description

By default, every TCP worker thread has its own queue, and incoming TCP connections are dispatched to TCP workers on a round-robin basis.
This might cause issues if some connections are taking a very long time, since incoming ones will be waiting until the TCP worker they have been assigned to has finished handling its current query, while other TCP workers might be available.
This experimental setTCPUseSinglePipe(true) directive can be used so that all the incoming TCP connections are put into a single queue and handled by thefirst TCP worker available.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code

[RobinGeuze]

Some small comments, but overal looks good. I'm curious how well a single pipe works when reading from it from multiple threads.

[RobinGeuze]

Bit pointless to leave this here with the constructor initialisation list in place.

[RobinGeuze]

Might be more consistent to preinitialize d_singlePipe to -1 for both, in that case you can do away with this else.