Implement a lazy set for the migration to the new node key format

[cool-develope]

Context

The migration to the new node key format is tricky. To avoid stopping the network, it introduces a lazy set. We will accept the existence of both new and old format nodes in the tree.

Proposal

• Refactor the Node structure to accept both old and new nodes, like this

// Node represents a node in a Tree.
type Node struct {
	key           []byte
	value         []byte
	hash          []byte
	nodeKey       *NodeKey
	leftNodeKey   []byte  // it can be a node hash for the old node and be a node key serialization for the new node.
	rightNodeKey  []byte
	size          int64
	leftNode      *Node
	rightNode     *Node
	subtreeHeight int8
}

• Setting new nodes will store the node as a new node key format. It will refactor them to the new format for old nodes that face while updating the tree.

[yihuang]

Sounds great, so there are just multiple versions of node key, new version could reference old version, but not the other way.

[cool-develope]

Sounds great, so there are just multiple versions of node key, new version could reference old version, but not the other way.

exactly