MINOR: listener: move thr_idx from the bind_conf to the listener

MINOR: listener: move thr_idx from the bind_conf to the listener

Tests show that it's slightly faster to have this field in the listener.
The cache walk patterns are under heavy stress and having only this field
written to in the bind_conf was wasting a cache line that was heavily
read. Let's move this close to the other entries already written to in
the listener. Warning, the position does have an impact on peak performance.