Bug 25723: Use the same code for single and exception holidays

Bug 25723: Use the same code for single and exception holidays

This patch refactors is_exception_holiday and is_single_holiday to
utilise a single _holidays method which combines the logic of the
previous single_holidays and exception_holidays methods.

As Koha::Calendar is instantiated at a branch level, we also move split
the cache into branches too.

Signed-off-by: Emma Perks <Emma.Perks2@uhb.nhs.uk>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>