[btcsuite/btcd] 781177: Implement BIP0065 changeover logic for v4 blocks.

[David Hill]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcd
  Commit: 7811770d3119ea4007df3167348ad6790930dad5
      
https://github.com/btcsuite/btcd/commit/7811770d3119ea4007df3167348ad6790930dad5
  Author: David Hill <dh...@mindcry.org>
  Date:   2015-10-28 (Wed, 28 Oct 2015)

  Changed paths:
    M blockchain/validate.go
    M mining.go
    M wire/blockheader.go

  Log Message:
  -----------
  Implement BIP0065 changeover logic for v4 blocks.

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