Server class code cleanups (#9769)

* Server::overrideDayNightRatio doesn't require to return bool
There is no sense to sending null player, the caller should send a valid object

* Server::init: make private & cleanup
This function is always called before start() and loads some variables which can be loaded in constructor directly.
Make it private and call it directly in start

* Split Server inventory responsibility to a dedicated object

This splits permit to found various historical issues:
* duplicate lookups on player connection
* sending inventory to non related player when a player connects
* non friendly lookups on detached inventories ownership

This reduce the detached inventory complexity and also increased the
lookup performance in a quite interesting way for servers with thousands
of inventories.

src/client/game.cpp

@@ -1302,7 +1302,6 @@ bool Game::createSingleplayerServer(const std::string &map_dir,
 	}
 
 	server = new Server(map_dir, gamespec, simple_singleplayer_mode, bind_addr, false);
-	server->init();
 	server->start();
 
 	return true;