consensus: improve consensus engine definition (#26871)

Makes clear the distinction between Finalize and FinalizedAndAssemble:

- In Finalize function, a series of state operations are applied according to consensus rules. The statedb is mutated and the root hash can be checked and compared afterwards.

This function should be used in block processing(receive afrom network and apply it locally) but not block generation.

- In FinalizeAndAssemble function, after applying state mutations, the block is also to be assembled with the latest
  state root computed, updating the header. 

 This function should be used in block generation only.

consensus/ethash/consensus.go

@@ -598,12 +598,10 @@ func (ethash *Ethash) Prepare(chain consensus.ChainHeaderReader, header *types.H
 	return nil
 }
 
-// Finalize implements consensus.Engine, accumulating the block and uncle rewards,
-// setting the final state on the header
+// Finalize implements consensus.Engine, accumulating the block and uncle rewards.
 func (ethash *Ethash) Finalize(chain consensus.ChainHeaderReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, withdrawals []*types.Withdrawal) {
-	// Accumulate any block and uncle rewards and commit the final state root
+	// Accumulate any block and uncle rewards
 	accumulateRewards(chain.Config(), state, header, uncles)
-	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))
 }
 
 // FinalizeAndAssemble implements consensus.Engine, accumulating the block and
@@ -612,9 +610,12 @@ func (ethash *Ethash) FinalizeAndAssemble(chain consensus.ChainHeaderReader, hea
 	if len(withdrawals) > 0 {
 		return nil, errors.New("ethash does not support withdrawals")
 	}
-
 	// Finalize block
 	ethash.Finalize(chain, header, state, txs, uncles, nil)
+
+	// Assign the final state root to header.
+	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))
+
 	// Header seems complete, assemble into a block and return
 	return types.NewBlock(header, txs, uncles, receipts, trie.NewStackTrie(nil)), nil
 }