Modify the key format of node in DB to a sequential integer ID instead of the hash value.

[cool-develope]

Context

As we discussed in #571, I am introducing the nonce mechanism here.

• add nodeKey field to replace the key of a node in DB
• replace a key in DB from node.hash to nodeKey
• investigate the possibility of removing version in the node structure

Design Decision

• immutable tree structure

type ImmutableTree struct {
	root                   *Node
	ndb                    *nodeDB
	version                int64
	nonce                  int64             // new field to track the current ID
	skipFastStorageUpgrade bool
}

• node structure

type Node struct {
	key           []byte
	value         []byte
	hash          []byte

	leftHash      []byte        
	rightHash     []byte    

	nodeKey       int64         // new field which reflects the current node key
	leftNodeKey   int64       // new field
	rightNodeKey  int64      // new field

	version       int64           // possible to remove ???

	size          int64
	leftNode      *Node
	rightNode     *Node
	subtreeHeight int8
	persisted     bool
}

[tac0turtle]

Before the final merge we should have an adr on the chnage

[cool-develope]

Before the final merge we should have an adr on the chnage

Yeah, I will prepare the adr when finishing all subtasks.

[catShaark]

I don't think we should remove leftHash and rightHash.
How can I collab with you on this ? I already had the code with the changes mentions here, I've modified methods like balance, delete, save branch ..... so that their logic matches with the new node struct, node saving mechanism.

[cool-develope]

@catShaark don't mind, please keep going your way. We'd like to compare the benchmark results of the two methods.