lightningd: have json_stream_success start the "result" object.

"result" should always be an object (so that we can add new fields),
so make that implicit in json_stream_success.

This makes our primitives well-formed: we previously used NULL as our
fieldname when calling the first json_object_start, which is a hack
since we're actually in an object and the fieldname is 'result' (which
was already written by json_object_start).

There were only two cases which didn't do this:
1. dev-memdump returned an array.  No API guarantees on this.
2. shutdown returned a string.

I temporarily made shutdown return an empty object, which shouldn't
break anything, but I want to fix that later anyway.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/test/run-invoice-select-inchan.c

@@ -291,9 +291,6 @@ void notify_connect(struct lightningd *ld UNNEEDED, struct node_id *nodeid UNNEE
 /* Generated stub for notify_disconnect */
 void notify_disconnect(struct lightningd *ld UNNEEDED, struct node_id *nodeid UNNEEDED)
 { fprintf(stderr, "notify_disconnect called!\n"); abort(); }
-/* Generated stub for null_response */
-struct json_stream *null_response(struct command *cmd UNNEEDED)
-{ fprintf(stderr, "null_response called!\n"); abort(); }
 /* Generated stub for onchaind_funding_spent */
 enum watch_result onchaind_funding_spent(struct channel *channel UNNEEDED,
 					 const struct bitcoin_tx *tx UNNEEDED,