Load all checkpoints during initialization

[stephank]

For things to work in litecoin / dogecoin, we need to load a pair of checkpoints, because the consensus algorithm requires us to look back a bit further: https://github.com/stephank/webcoin-dogecoin/blob/3f2f7bbcb8be9e53126a1e35fdaf5d55f632617f/index.js#L113-L116

I also have a patch for 2.x, because that's what webcoin still uses, but there's no branch to send the pull request to: https://github.com/mappum/blockchain-spv/compare/v2.1.0...stephank:2.x