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/clique/clique.go

@@ -565,12 +565,10 @@ func (c *Clique) Prepare(chain consensus.ChainHeaderReader, header *types.Header
 	return nil
 }
 
-// Finalize implements consensus.Engine, ensuring no uncles are set, nor block
-// rewards given.
+// Finalize implements consensus.Engine. There is no post-transaction
+// consensus rules in clique, do nothing here.
 func (c *Clique) Finalize(chain consensus.ChainHeaderReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, withdrawals []*types.Withdrawal) {
-	// No block rewards in PoA, so the state remains as is and uncles are dropped
-	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))
-	header.UncleHash = types.CalcUncleHash(nil)
+	// No block rewards in PoA, so the state remains as is
 }
 
 // FinalizeAndAssemble implements consensus.Engine, ensuring no uncles are set,
@@ -579,11 +577,13 @@ func (c *Clique) FinalizeAndAssemble(chain consensus.ChainHeaderReader, header *
 	if len(withdrawals) > 0 {
 		return nil, errors.New("clique does not support withdrawals")
 	}
-
 	// Finalize block
 	c.Finalize(chain, header, state, txs, uncles, nil)
 
-	// Assemble and return the final block for sealing
+	// Assign the final state root to header.
+	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))
+
+	// Assemble and return the final block for sealing.
 	return types.NewBlock(header, txs, nil, receipts, trie.NewStackTrie(nil)), nil
 }