Bug 33837: Add datetime comparison to filter_by_last_update

Bug 33837: Add datetime comparison to filter_by_last_update

Test plan:
Verify that filter_by_last_update still works as expected by
running cleanup_database.pl with --messages DAYS parameter.
Look at the oldest messages.message_date (or insert one).
Run with -days X where today - X is before that date.
Run with -days Y where today - Y is just after it.

Note: misc/cronjobs/cleanup_database.pl -confirm -messages -1 -v
will remove all messages including one you inserted just now.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>