plugin: Cleanup a plugin as soon as its stdout closes

We were waiting for both stdin and stdout to close, however that resulted in
us deferring cleanup indefinitely since we did not poll stdout for being
writable most of the time. On the other hand we are almost always polling
the plugin's stdout, so that notifies us as soon as the plugin stops.

Changelog-Fixed: plugin: Plugins no longer linger indefinitely if their process terminates

lightningd/plugin.c

@@ -439,14 +439,8 @@ static struct io_plan *plugin_write_json(struct io_conn *conn,
  */
 static void plugin_conn_finish(struct io_conn *conn, struct plugin *plugin)
 {
-	if (conn == plugin->stdin_conn)
-		plugin->stdin_conn = NULL;
-
-	else if (conn == plugin->stdout_conn)
-		plugin->stdout_conn = NULL;
-
-	if (plugin->stdin_conn == NULL && plugin->stdout_conn == NULL)
-		tal_free(plugin);
+	plugin->stdout_conn = NULL;
+	tal_free(plugin);
 }
 
 struct io_plan *plugin_stdin_conn_init(struct io_conn *conn,
@@ -454,8 +448,8 @@ struct io_plan *plugin_stdin_conn_init(struct io_conn *conn,
 {
 	/* We write to their stdin */
 	/* We don't have anything queued yet, wait for notification */
+	plugin->stdin_conn = tal_steal(plugin, conn);
 	plugin->stdin_conn = conn;
-	io_set_finish(conn, plugin_conn_finish, plugin);
 	return io_wait(plugin->stdin_conn, plugin, plugin_write_json, plugin);
 }