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/types/queue.h b/include/types/queue.h
index 575cc59..361c704 100644
--- a/include/types/queue.h
+++ b/include/types/queue.h
@@ -37,7 +37,7 @@
 	struct proxy  *px;
 	struct server *srv;        /* the server we are waiting for, may be NULL if don't care */
 	struct server *target;     /* the server that was assigned, = srv except if srv==NULL */
-	struct list    list;       /* next pendconn */
+	struct eb32_node node;
 };
 
 #endif /* _TYPES_QUEUE_H */