PPCoin: Relocate synchronized checkpoint code to checkpointsync.cpp
        -checkpointenforce configuration to allow user to opt out checkpoint
        enforcecheckpoint RPC command to change the above configuration
        Checkpoint operator tools: -checkpointdepth, sendcheckpoint
        Use base58 private key for checkpoint master key
        Checkpoint operator tools: makekeypair, showkeypair
        Testnet checkpoint master key