blockchain: Optimize old block ver upgrade checks.

Currently the code that rejects old block versions once the majority of
the network has upgraded is performed using a loop that iterates
backwards from latest enforced block version while checking for a
majority upgrade at each version.  This is inefficient for blocks early
in the chain since it means multiple versions need to be checked when it
isn't really necessary.

This optimizes the relevant code by taking into account that the
relevant enforcement semantics reduce to the following:

- Block versions greater than or equal to the latest enforced block
  version can never be rejected for being old
- Block versions must be rejected when the majority of the network has
  upgraded to ANY version greater than that version

The following timing information obtained from profiling shows the
overall reduction achieved with the optimized code for mainnet:

 existing: Total time to check old versions for 645603 headers: 10.61118s
optimized: Total time to check old versions for 645603 headers:  3.52555s

blockchain/validate.go

@@ -988,10 +988,12 @@ func CheckBlockSanity(block *dcrutil.Block, timeSource MedianTimeSource, chainPa
 func isDCP0005Violation(network wire.CurrencyNet, header *wire.BlockHeader, blockHash *chainhash.Hash) bool {
 	switch network {
 	case wire.MainNet:
-		// 430191 is the height of the last block on mainnet that violated
-		// DCP0005, so return false immediately if the height is greater than
-		// that.
-		if header.Height > 430191 {
+		// All blocks that violated DCP0005 on mainnet are version 6 and the
+		// height of the last block that violated it is 430191.
+		//
+		// Avoid additional work for blocks that are either not that version or
+		// are after the final height known to be a violation.
+		if header.Version != 6 || header.Height > 430191 {
 			return false
 		}
 
@@ -1004,16 +1006,50 @@ func isDCP0005Violation(network wire.CurrencyNet, header *wire.BlockHeader, bloc
 			header.Height == 430191 && *blockHash == *block430191Hash
 
 	case wire.TestNet3:
-		// 323282 is the height of the last block on testnet that violated
-		// DCP0005.  Return true if the height is less than or equal to 323282
-		// so that the DCP0005 version check is not enforced until after that
-		// point.
-		return header.Height <= 323282
+		// All blocks that violated DCP0005 on testnet are version 7 and the
+		// height of the last block that violated it is 323282.
+		//
+		// Return true for all blocks of that version that are before the final
+		// height known to be a violation so that the DCP0005 version check is
+		// not enforced until after that point.
+		return header.Version == 7 && header.Height <= 323282
 	}
 
 	return false
 }
 
+// isOldBlockVersionByMajority determines if the block version specified by the
+// provided header should be rejected due to a majority of the network already
+// being upgraded to a newer version.
+//
+// This function MUST be called with the chain state lock held (for reads).
+func (b *BlockChain) isOldBlockVersionByMajority(header *wire.BlockHeader, blockHash *chainhash.Hash, prevNode *blockNode) bool {
+	// Note that the latest block version for all networks other than the main
+	// network is one higher.
+	latestBlockVersion := int32(9)
+	if b.chainParams.Net != wire.MainNet {
+		latestBlockVersion++
+	}
+
+	// Blocks with a version greater than or equal to the latest enforced block
+	// version are most certainly not an old version.
+	if header.Version >= latestBlockVersion {
+		return false
+	}
+
+	// Skip the version check for blocks that are known to violate DCP0005.  See
+	// isDCP0005Violation for more details.
+	if isDCP0005Violation(b.chainParams.Net, header, blockHash) {
+		return false
+	}
+
+	// The block version is considered old once the majority of the network has
+	// upgraded to a more recent version.
+	rejectNumRequired := b.chainParams.BlockRejectNumRequired
+	nextVersion := header.Version + 1
+	return b.isMajorityVersion(nextVersion, prevNode, rejectNumRequired)
+}
+
 // checkBlockHeaderPositional performs several validation checks on the block
 // header which depend on its position within the block chain and having the
 // headers of all ancestors available.  These checks do not, and must not, rely
@@ -1092,31 +1128,10 @@ func (b *BlockChain) checkBlockHeaderPositional(header *wire.BlockHeader, prevNo
 	if !fastAdd {
 		// Reject old version blocks once a majority of the network has
 		// upgraded.
-		//
-		// Note that the latest block version for all networks other than the
-		// main network is one higher.
-		latestBlockVersion := int32(9)
-		dcp0005Version := int32(7)
-		if b.chainParams.Net != wire.MainNet {
-			latestBlockVersion++
-			dcp0005Version++
-		}
-		for version := latestBlockVersion; version > 1; version-- {
-			// Skip the version check for blocks that are known to violate
-			// DCP0005.  See isDCP0005Violation for more details.
-			if version == dcp0005Version &&
-				isDCP0005Violation(b.chainParams.Net, header, &blockHash) {
-
-				continue
-			}
-
-			if header.Version < version && b.isMajorityVersion(version,
-				prevNode, b.chainParams.BlockRejectNumRequired) {
-
-				str := "new blocks with version %d are no longer valid"
-				str = fmt.Sprintf(str, header.Version)
-				return ruleError(ErrBlockVersionTooOld, str)
-			}
+		if b.isOldBlockVersionByMajority(header, &blockHash, prevNode) {
+			str := "new blocks with version %d are no longer valid"
+			str = fmt.Sprintf(str, header.Version)
+			return ruleError(ErrBlockVersionTooOld, str)
 		}
 	}