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

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

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 the
first TCP worker available.