[MINOR] proxy: add a "parent" member to the structure

This member will be used later when frontends are created on the
fly by some tasks. It will also be usable later if we need to
support multiple config instances for example.

include/types/proxy.h

@@ -299,6 +299,7 @@ struct proxy {
 		struct eb_root used_listener_id;/* list of listener IDs in use */
 		struct eb_root used_server_id;	/* list of server IDs in use */
 	} conf;					/* config information */
+	void *parent;				/* parent of the proxy when applicable */
 };
 
 struct switching_rule {