db/Plugin: use std::unique_ptr<> to manage Database pointers

src/Main.cxx

@@ -188,7 +188,7 @@ glue_db_init_and_load(const ConfigData &config)
 	instance->database =
 		CreateConfiguredDatabase(config, instance->event_loop,
 					 instance->io_thread.GetEventLoop(),
-					 *instance);
+					 *instance).release();
 	if (instance->database == nullptr)
 		return true;