MEDIUM: listeners: split the thread mask between receiver and bind_conf

With groups at some point we'll have to have distinct masks/groups in the
receiver and the bind_conf, because a single bind_conf might require to
instantiate multiple receivers (one per group).

Let's split the thread mask and group to have one for the bind_conf and
another one for the receiver while it remains easy to do. This will later
allow to use different storage for the bind_conf if needed (e.g. support
multiple groups).
diff --git a/src/sock_inet.c b/src/sock_inet.c
index fd8f648..fa04dfe 100644
--- a/src/sock_inet.c
+++ b/src/sock_inet.c
@@ -391,7 +391,7 @@
 	rx->fd = fd;
 	rx->flags |= RX_F_BOUND;
 
-	fd_insert(fd, rx->owner, rx->iocb, thread_mask(rx->settings->bind_thread) & all_threads_mask);
+	fd_insert(fd, rx->owner, rx->iocb, thread_mask(rx->bind_thread) & all_threads_mask);
 
 	/* for now, all regularly bound TCP listeners are exportable */
 	if (!(rx->flags & RX_F_INHERITED))