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.

doc/lua_api.txt

@@ -4240,6 +4240,13 @@ Definition tables
           on ground when the player places the item. Server will always update
           actual result to client in a short moment.
         ]]
+        node_dig_prediction = "air",
+        --[[
+        ^ if "", no prediction is made
+        ^ if "air", node is removed
+        ^ Otherwise should be name of node which the client immediately places
+          upon digging. Server will always update actual result shortly.
+        ]]
         sound = {
             breaks = "default_tool_break", -- tools only
             place = --[[<SimpleSoundSpec>]],