consensus,core,miner: avoid overhead of creating a new block

[holiman]

This PR removes some overhead work done during block import, where we uselessly assemble a block which is then thrown away.

In state processor

	// Finalize the block, applying any consensus engine specific extras (e.g. block rewards)
	p.engine.Finalize(p.bc, header, statedb, block.Transactions(), block.Uncles(), receipts)

ethash/consensus.go:

// Finalize implements consensus.Engine, accumulating the block and uncle rewards,
// setting the final state and assembling the block.
func (ethash *Ethash) Finalize(chain consensus.ChainReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, receipts []*types.Receipt) (*types.Block, error) {
	// Accumulate any block and uncle rewards and commit the final state root
	accumulateRewards(chain.Config(), state, header, uncles)
	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), nil
}

The Finalize returns a new types.Block, which copies the entire block and recalculates the transcation hash, unclehash, receipt hash and log bloom. The returned new block is totally ignored by the state processor, however.

This PR skips that recalculation. That calculation of hashes and verification against the given block is (already) performed later, in ValidateState.

[karalabe]

LGTM