core: clone MPT node while restoring it multiple times

We need this to avoid collapse collisions. Example of such collapse
described in
https://github.com/nspcc-dev/neo-go/pull/2019#discussion_r689629704.

pkg/core/mpt/node.go

@@ -34,6 +34,7 @@ type Node interface {
 	json.Marshaler
 	json.Unmarshaler
 	Size() int
+	Clone() Node
 	BaseNodeIface
 }