Feature: Allow checksum to only compare diffs from previous runs

It'd be nice if you could run pt-table-checksum multiple times, against a prior result.

An example scenario: a large table has been checksummed, discrepencies were found. User runs pt-table-sync to address the discrepencies, but must cancel pt-table-sync mid-run. It'd be nice if you could call pt-table-checksum with a flag that would only re-checksum the chuncks that showed a difference, and updated them accordingly.

A full pt-table-checkum on some of my tables can take upwards of 8 hours, without taking slave lag into consideration. It'd be nice if I didn't have to re-checksum the whole thing in between individiual pt-table-sync runs, but also not have pt-table-checksum compare a bunch of diffs that may no longer be diffs.