core/rawdb, core/state: polish the transition state implementation

[rjl493456442]

In this pull request, several changes have been made:

(a) Replaced CurrentAccountAddress *common.Address with CurrentAccountHash common.Hash

Managing a nullable pointer adds unnecessary complexity. This change simplifies the
logic by replacing the pointer with a value based flag:

• Previously: CurrentAccountAddress == nil indicated that no account had been migrated yet.
• Now: CurrentAccountHash == common.Hash{} serves the same purpose.

This approach is already used in CurrentSlotHash, so this change aligns the handling of both fields.

(b) Removed LoadTransitionState call sites from the trie reader

The current usage of LoadTransitionState in the trie reader is not correct. We should consider
five different scenarios:

(a) TransitionState(root) is not found:
Verkle is not activated yet => use merkle tree

(b) TransitionState(root) is not found and ChainConfig.IsVerkle(blockNumber, blockTime) is true:
The first Verkle block at the boundary => use overlay tree

(c) ChainConfig.IsVerkle(blockNumber, blockTime) is true and TransitionState(root).Started is false:
The verkle block before the  migration => use overlay tree

(d) ChainConfig.IsVerkle(blockNumber, blockTime) is true and TransitionState(root).Started is true:
The verkle block during the migration => use overlay tree

(e)  ChainConfig.IsVerkle(blockNumber, blockTime) is true and TransitionState(root).Ended is true
Verkle transition completes => use verkle tree

However, in your implementation, e.g., in the code below, it's wrong. As you mentioned,
the migration is only started if the base merkle state if finalized. !ts.InTransition() && !ts.Transitioned()
can refer to the situation that Verkle has been activated but the migration is not. In this
case, Overlay tree should be used instead of Merkle here. We should separate this part
of logic into a following PR.

// OpenStorageTrie opens the storage trie of an account.
func (db *CachingDB) OpenStorageTrie(stateRoot common.Hash, address common.Address, root common.Hash, self Trie) (Trie, error) {
	ts := db.LoadTransitionState(stateRoot)
	// In the verkle case, there is only one tree. But the two-tree structure
	// is hardcoded in the codebase. So we need to return the same trie in this
	// case.
	if ts.InTransition() || ts.Transitioned() {
		return self, nil
	}
	tr, err := trie.NewStateTrie(trie.StorageTrieID(stateRoot, crypto.Keccak256Hash(address.Bytes()), root), db.triedb)
	if err != nil {
		return nil, err
	}
	return tr, nil
}

[rjl493456442]

&overlay.TransitionState{Ended: true}) is apparently wrong here. Let's remove it for now.

[rjl493456442]

Duplicated with the one in genesis.Commit(db, triedb). It's also not correct to persist a transition-state with zero root.

[rjl493456442]

It's unnecessary to persist any transition flag, as this function is supposed to calculate the state root of genesis only.

[rjl493456442]

Duplicated with the one in genesis.Commit(db, triedb). It's also not correct to persist a transition-state with zero root.