Make histogram samples non-cumulative by tracking a linked-list

Make histogram samples non-cumulative by tracking a linked-list
of the most recent histogram and differencing it when we print
to the log file(s). Linked list of pointers used to minimize
runtime impact on recording side, instead choosing to do
subtraction on the logging (when logs get printed to file) side.

This helps cleanup fiologparser_hist.py, makes the log files easier
to understand at a glance, reduces file size(s), and averts the
possibility of integer overflow in the logs (for a long enough fio
job).