Implement client node dig prediction

Dig prediction allows clients to remove dug nodes without
waiting for server acknowledgement.
This patch allows mods to override dig prediction,
it can either be turned off or a different "prediction node"
can be selected.

src/nodedef.h

@@ -324,6 +324,8 @@ struct ContentFeatures
 	// Player cannot build to these (placement prediction disabled)
 	bool rightclickable;
 	u32 damage_per_second;
+	// client dig prediction
+	std::string node_dig_prediction;
 
 	// --- LIQUID PROPERTIES ---