BuildCheckpoints: create checkpoint files to use with CheckpointManager

Usage: build-checkpoints --flags

>>> OPTIONS
  --net=XXX            Which network to connect to, defaults to MAIN, can also be TEST or REGTEST.
  --peer=1.2.3.4       IP address/domain name for connection instead of localhost.
  --days=<int>         How many days to keep as a safety margin. Checkpointing will be done up to this many days ago.