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/server.h b/include/types/server.h
index 7d0ba45..8825928 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -215,7 +215,7 @@
 	struct freq_ctr sess_per_sec;		/* sessions per second on this server */
 	struct be_counters counters;		/* statistics counters */
 
-	struct list pendconns;			/* pending connections */
+	struct eb_root pendconns;		/* pending connections */
 	struct list actconns;			/* active connections */
 	struct list *priv_conns;		/* private idle connections attached to stream interfaces */
 	struct list *idle_conns;		/* sharable idle connections attached or not to a stream interface */