Refactor bitcask_fileops:fold_keys function

Refactor bitcask_fileops:fold_keys to remove nested case statements and
improve the clarity. Also change the has_valid_hintfile function to only check
for a valid hintfile and not also check for the existence of a hintfile. This
change has the side effect of preventing the logging of error messages about
invalid hintfiles when the case is just that no hintfile exists.