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

src/db/Configured.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -20,10 +20,11 @@
 #ifndef MPD_DB_CONFIG_HXX
 #define MPD_DB_CONFIG_HXX
 
+#include "Ptr.hxx"
+
 struct ConfigData;
 class EventLoop;
 class DatabaseListener;
-class Database;
 
 /**
  * Read database configuration settings and create a #Database
@@ -32,7 +33,7 @@ class Database;
  *
  * Throws #std::runtime_error on error.
  */
-Database *
+DatabasePtr
 CreateConfiguredDatabase(const ConfigData &config,
 			 EventLoop &main_event_loop, EventLoop &io_event_loop,
 			 DatabaseListener &listener);