Implement BIP0065 changeover logic for v4 blocks.

This commit implements the changeover logic for version 4 blocks as
described by BIP0065.

blockchain/validate.go

@@ -653,6 +653,16 @@ func (b *BlockChain) checkBlockHeaderContext(header *wire.BlockHeader, prevNode
 	}
 
 	if !fastAdd {
+		// Reject version 3 blocks once a majority of the network has
+		// upgraded.  This is part of BIP0065.
+		if header.Version < 4 && b.isMajorityVersion(4, prevNode,
+			b.chainParams.BlockRejectNumRequired) {
+
+			str := "new blocks with version %d are no longer valid"
+			str = fmt.Sprintf(str, header.Version)
+			return ruleError(ErrBlockVersionTooOld, str)
+		}
+
 		// Reject version 2 blocks once a majority of the network has
 		// upgraded.  This is part of BIP0066.
 		if header.Version < 3 && b.isMajorityVersion(3, prevNode,
@@ -1133,6 +1143,15 @@ func (b *BlockChain) checkConnectBlock(node *blockNode, block *btcutil.Block) er
 		scriptFlags |= txscript.ScriptVerifyDERSignatures
 	}
 
+	// Enforce CHECKLOCKTIMEVERIFY for block versions 4+ once the majority
+	// of the network has upgraded to the enforcement threshold.  This is
+	// part of BIP0065.
+	if blockHeader.Version >= 4 && b.isMajorityVersion(4, prevNode,
+		b.chainParams.BlockEnforceNumRequired) {
+
+		scriptFlags |= txscript.ScriptVerifyCheckLockTimeVerify
+	}
+
 	// Now that the inexpensive checks are done and have passed, verify the
 	// transactions are actually allowed to spend the coins by running the
 	// expensive ECDSA signature check scripts.  Doing this last helps

mining.go

@@ -24,7 +24,7 @@ const (
 	// will require changes to the generated block.  Using the wire constant
 	// for generated block version could allow creation of invalid blocks
 	// for the updated version.
-	generatedBlockVersion = 3
+	generatedBlockVersion = 4
 
 	// minHighPriority is the minimum priority value that allows a
 	// transaction to be considered high priority.

wire/blockheader.go

@@ -11,7 +11,7 @@ import (
 )
 
 // BlockVersion is the current latest supported block version.
-const BlockVersion = 3
+const BlockVersion = 4
 
 // MaxBlockHeaderPayload is the maximum number of bytes a block header can be.
 // Version 4 bytes + Timestamp 4 bytes + Bits 4 bytes + Nonce 4 bytes +