CLEANUP: listener: only store conn counts for local threads

CLEANUP: listener: only store conn counts for local threads

The listeners have a thr_conn[] array indexed on the thread number that
is used during connection redispatching to know what threads are the least
loaded. Since we introduced thread groups, and based on the fact that a
listener may only belong to one group, there's no point storing counters
for all threads, we just need to store them for all threads in the group.

Doing so reduces the struct listener from 1500 to 632 bytes. This may be
backported to 2.7 to save a bit of resources.