lightningd: log IO only on actual output.

This causes natural batching, rather than on every little addition of
JSON formatting.

Before, to listchannels 100,000 channels took 82.48 seconds, after
6.82 seconds.

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

lightningd/json_stream.c

@@ -116,8 +116,6 @@ static void js_written_some(struct json_stream *js)
 void json_stream_append_part(struct json_stream *js, const char *str, size_t len)
 {
 	mkroom(js, len);
-	if (js->log)
-		log_io(js->log, LOG_IO_OUT, "", str, len);
 	memcpy(membuf_add(&js->outbuf, len), str, len);
 	js_written_some(js);
 }
@@ -148,9 +146,6 @@ static void json_stream_append_vfmt(struct json_stream *js,
 		mkroom(js, fmtlen + 1);
 		vsprintf(membuf_space(&js->outbuf), fmt, ap2);
 	}
-	if (js->log)
-		log_io(js->log, LOG_IO_OUT, "",
-		       membuf_space(&js->outbuf), fmtlen);
 	membuf_added(&js->outbuf, fmtlen);
 	js_written_some(js);
 	va_end(ap2);
@@ -296,6 +291,9 @@ static struct io_plan *json_stream_output_write(struct io_conn *conn,
 	}
 
 	js->reader = conn;
+	if (js->log)
+		log_io(js->log, LOG_IO_OUT, "",
+		       membuf_elems(&js->outbuf), js->len_read);
 	return io_write(conn,
 			membuf_elems(&js->outbuf), js->len_read,
 			json_stream_output_write, js);