All functions in src/cryptonote_core/checkpoints.h are now documented in
doxygen style.
checkpoints.cpp has been reviewed, one function has been marked for
discussion on correctness.
json checkpoints will be checked every 10 minutes, dns every 60.
json checkpoints always enforced, dns still with flag.
conflicting checkpoints is hard fail, but soft if dns enforce flag not
set and dns checkpoints are wonky.
For checkpoints being read at runtime to work correctly, the checkpoint
add code needs to not return false if a checkpoint is added that already
exists. In this case, instead return false if the checkpoint is for a
height that already has a checkpoint and the hashes are different.
