Bug 16044: Add an unsafe flag to Koha::Cache->get_from_cache

 Bug 16044: Add an unsafe flag to Koha::Cache->get_from_cache

If the caller/developer knows what he is doing, he can decide not to
deep copy the structure. It will be faster but unsafe!
If the structure is modified, the cache will also be updated.
This option must be used with care and is not the default behavior.

Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>