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

All three elements (pendconns, nbpend, queue_idx) were moved to struct
queue.
diff --git a/src/proxy.c b/src/proxy.c
index 5a7f103..f6d0442 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1292,7 +1292,7 @@
 {
 	memset(p, 0, sizeof(struct proxy));
 	p->obj_type = OBJ_TYPE_PROXY;
-	p->pendconns = EB_ROOT;
+	p->queue.head = EB_ROOT;
 	LIST_INIT(&p->acl);
 	LIST_INIT(&p->http_req_rules);
 	LIST_INIT(&p->http_res_rules);