MINOR: queue: replace the linked list with a tree

We'll need trees to manage the queues by priorities. This change replaces
the list with a tree based on a single key. It's effectively a list but
allows us to get rid of the list management right now.
diff --git a/include/proto/queue.h b/include/proto/queue.h
index 11696db..166da12 100644
--- a/include/proto/queue.h
+++ b/include/proto/queue.h
@@ -52,7 +52,7 @@
  */
 static inline void pendconn_cond_unlink(struct pendconn *p)
 {
-	if (p && !LIST_ISEMPTY(&p->list))
+	if (p && p->node.node.leaf_p)
 		pendconn_unlink(p);
 }