libplugin: add a 'still_pending' helper

plugins/libplugin.c

@@ -217,6 +217,12 @@ command_finished(struct command *cmd, struct json_stream *response)
 	return command_complete(cmd, response);
 }
 
+struct command_result *WARN_UNUSED_RESULT
+command_still_pending(struct command *cmd)
+{
+	return &pending;
+}
+
 struct json_out *json_out_obj(const tal_t *ctx,
 			      const char *fieldname,
 			      const char *str)

plugins/libplugin.h

@@ -138,6 +138,10 @@ struct json_stream *jsonrpc_stream_fail_data(struct command *cmd,
 struct command_result *WARN_UNUSED_RESULT
 command_finished(struct command *cmd, struct json_stream *response);
 
+/* Helper for a command that'll be finished in a callback. */
+struct command_result *WARN_UNUSED_RESULT
+command_still_pending(struct command *cmd);
+
 /* Helper to create a zero or single-value JSON object; if @str is NULL,
  * object is empty. */
 struct json_out *json_out_obj(const tal_t *ctx,