Monoblocks: optimize blocks that contain a single type of node (#16293)

Reduces memory usage on the server, especially with many user and/or large viewing distances.
Currently disabled on the client due to known data races on a block's data.

src/client/client.h

@@ -371,6 +371,7 @@ public:
 	scene::ISceneManager *getSceneManager();
 
 	// IGameDef interface
+	bool isClient() override { return true; }
 	IItemDefManager* getItemDefManager() override;
 	const NodeDefManager* getNodeDefManager() override;
 	ICraftDefManager* getCraftDefManager() override;