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_fas.c b/src/lb_fas.c
index 53bd039..d90388b 100644
--- a/src/lb_fas.c
+++ b/src/lb_fas.c
@@ -322,7 +322,7 @@
 		struct server *s;
 
 		s = eb32_entry(node, struct server, lb_node);
-		if (!s->maxconn || (!s->nbpend && s->served < srv_dynamic_maxconn(s))) {
+		if (!s->maxconn || (!s->queue.length && s->served < srv_dynamic_maxconn(s))) {
 			if (s != srvtoavoid) {
 				srv = s;
 				break;