MINOR: server: replace the pendconns-related stuff with a struct queue

Just like for proxies, all three elements (pendconns, nbpend, queue_idx)
were moved to struct queue.
diff --git a/src/lb_fwrr.c b/src/lb_fwrr.c
index d7f618f..74c7fb2 100644
--- a/src/lb_fwrr.c
+++ b/src/lb_fwrr.c
@@ -564,7 +564,7 @@
 		fwrr_update_position(grp, srv);
 		fwrr_dequeue_srv(srv);
 		grp->curr_pos++;
-		if (!srv->maxconn || (!srv->nbpend && srv->served < srv_dynamic_maxconn(srv))) {
+		if (!srv->maxconn || (!srv->queue.length && srv->served < srv_dynamic_maxconn(srv))) {
 			/* make sure it is not the server we are trying to exclude... */
 			if (srv != srvtoavoid || avoided)
 				break;