// this file handles and maintains checkpoints which are used by blockchain to skip some checks on known parts of the chain
// the skipping of the checks can be disabled by command line arguments

根据：高度 + hash 进行判断。指定具体高度，其 hash 值，与给定预设的值是否相同？

像中本聪共识这种取最长链规则的，还可以用此拒绝 51% 攻击。

即，不能推翻历史（已完全确认的区块链数据）。即使矿工有超过 51% 算力，即使矿工产生新的最长链。

LoadCheckPoints

// load checkpoints from the data directory
// a line should be printed on console when we are doing this

IsCheckSumKnown

// tell whether a checkpoint is known in the current selected network