dbft: allow to apply Eth-based block verification for unsigned block

To be able to properly verify unsigned block after PrepareRequest receival
in WithVerifyBlock dBFT callback we need to ensure that (*DBFT).Author can
handle such unsugned block. Starting from the previous commit, we define
block's Coinbase as the Primary address for the dBFT round when the block
was accepted. And thus, return proper block author for proper verification.

The problem itself is described in #87 (comment).
Also ref. #51.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

Author: AnnaShaleva

consensus/dbft/dbft.go

@@ -635,13 +635,29 @@ func (c *DBFT) postBlock(b *types.Block) {
 }
 
 // Author implements consensus.Engine, returning the Ethereum address recovered
-// from the signature in the header's extra-data section.
+// from the header's Coinbase.
 func (c *DBFT) Author(header *types.Header) (common.Address, error) {
-	vals, _, err := getSignersAndSigs(c.config, header.Extra)
-	if err != nil {
-		return common.Address{}, fmt.Errorf("failed to retrieve validators addresses and signatures from header: %w", err)
-	}
-	return vals[header.Primary()], nil
+	// TODO: we must agree on this with @roman-khimov and @chenquanyu. Block author
+	// must be defined by the moment of WithVerifyBlock dBFT callback call, thus,
+	// can't depend on changeable block's Extra field. At the same time, block author
+	// must correspond to miner.etherbase setting used to run node for proper block
+	// verification and reward distribution. From my POW, we have two options to
+	// provide these constraints:
+	// 1. For each consensus node, in the node config enforce miner.etherbase be
+	//    a corresponding consensus node address, make different miner.etherbase
+	//    no-op for DBFT engine. Then, miner will submit task with coinbase (that's
+	//    the same as miner.coinbase) to dBFT and dBFT will manage coinbase internally
+	//    and set it to the Primary of the current dBFT round. WithNewBlockFromContext
+	//    will provide this. Given this fact, Coinbase is always the address of Primary
+	//    that was the block's author or the accepted block at the current height. And
+	//    then Coinbase then can be used for reward calculations.
+	// 2. If Coinbase is expected to have another meaning (i.e. it's expected to be
+	//    some side address), then we must share the rules of Coinbase detection between
+	//    consensus nodes. In this case we need to decide what's the desired meaning of Coinbase.
+	//
+	// Currently option 1 is implemented, but without enforced miner.etherbase check. Please,
+	// write your thoughts on this.
+	return header.Coinbase, nil
 }
 
 // VerifyHeader checks whether a header conforms to the consensus rules.