Add 'disable_checkpoints' config setting, refactor checkpoint code

[who-biz]

This commit adds a config file option in server settings called disable_checkpoints

Fast Sync with pow skipping now covers three different usage cases:

1. User sets ONLY skip_pow_validation = true

   • results in skipping of pow validation on all blocks up to last checkpoint (2,050,000 in source)

2. User sets skip_pow_validation = true AND disable_checkpoints = true

   • results in fast sync all the way to chaintip (pow validation begins only once we've synced all historical blocks)

3. User sets neither

   • results in full-chain validation.

This also required some code refactoring to accommodate the new mechanics, and config file access through inheritance. I actually like this implementation a lot better, as it is easier to follow than the previous.

Finally, it resolves an omission I made, in original submission of skip_pow_validation functionality (not sure it would have made a difference, but its resolved here also)

[who-biz]

Small hack for quicker testing: comment out the newer checkpoints and recompile... same effect willl trigger at whichever height is included in the last checkpoint.