Bug 15333: Use Koha::Cache to cache exception_holidays instead of a package variable

Bug 15333: Use Koha::Cache to cache exception_holidays instead of a package variable

On the same way as bug 14522, we should use Koha::Cache to cache
exception_holidays.
It's not safe to use a package variable if running under Plack.

There is not test plan, just make sure the changes make sense.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit cbd375fab76a89b75f3795c9ebafe434bf79c878)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>