Avoid copying block headers in convenience locals.

This commit modifies local variables that are used for more convenient
access to a block's header to use pointers.  This avoids copying the
header multiple times.

accept.go

@@ -34,8 +34,7 @@ func (b *BlockChain) maybeAcceptBlock(block *btcutil.Block, fastAdd bool) error
 	}
 	block.SetHeight(blockHeight)
 
-	blockHeader := block.MsgBlock().Header
-
+	blockHeader := &block.MsgBlock().Header
 	if !fastAdd {
 		// Ensure the difficulty specified in the block header matches
 		// the calculated difficulty based on the previous block and

process.go

@@ -123,14 +123,14 @@ func (b *BlockChain) ProcessBlock(block *btcutil.Block, fastAdd bool) error {
 	// easy to mine, but otherwise bogus, blocks that could be used to eat
 	// memory, and ensuring expected (versus claimed) proof of work
 	// requirements since the last checkpoint are met.
-	blockHeader := block.MsgBlock().Header
+	blockHeader := &block.MsgBlock().Header
 	checkpointBlock, err := b.findLatestKnownCheckpoint()
 	if err != nil {
 		return err
 	}
 	if checkpointBlock != nil {
 		// Ensure the block timestamp is after the checkpoint timestamp.
-		checkpointHeader := checkpointBlock.MsgBlock().Header
+		checkpointHeader := &checkpointBlock.MsgBlock().Header
 		checkpointTime := checkpointHeader.Timestamp
 		if blockHeader.Timestamp.Before(checkpointTime) {
 			str := fmt.Sprintf("block %v has timestamp %v before "+

validate.go

@@ -268,8 +268,7 @@ func CheckTransactionSanity(tx *btcutil.Tx) error {
 // target difficulty as claimed.
 func checkProofOfWork(block *btcutil.Block, powLimit *big.Int) error {
 	// The target difficulty must be larger than zero.
-	header := block.MsgBlock().Header
-	target := CompactToBig(header.Bits)
+	target := CompactToBig(block.MsgBlock().Header.Bits)
 	if target.Sign() <= 0 {
 		str := fmt.Sprintf("block target difficulty of %064x is too low",
 			target)