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.
diff --git a/include/types/connection.h b/include/types/connection.h
index 1fa0b73..998ecac 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -52,6 +52,7 @@
 struct wait_list {
 	struct tasklet *task;
 	struct list list;
+	void *handle;           /* To be used by the callee */
 	int wait_reason;
 };