Add an option to pmp-check-pt-table-checksum to check MAX(ts) of latest checksum

While checking checksums is very helpful and needed, we should also have a flag that checks MAX(ts) FROM percona.checksums and ensure it is within some acceptable period (say within last 24 hrs or 7 days).

Use case:
It would be unfortunate for the cron job that runs pt-table-checksum to get stopped and we never discover data set differences because last run of pt-table-checksum from weeks ago was clean.