MINOR: connections: Add a "handle" field to wait_list.

Add a new field to struct wait_list, "handle", that can be used by the
entity in charge of subscribing.

include/types/connection.h

@@ -52,6 +52,7 @@ enum sub_event_type {
 struct wait_list {
 	struct tasklet *task;
 	struct list list;
+	void *handle;           /* To be used by the callee */
 	int wait_reason;
 };