Bug 11998: Add a L1 cache for sysprefs

Bug 11998: Add a L1 cache for sysprefs

Accessing to the cache for each call to C4::Context->preference might
have an impact on performances.
To avoid that this patch introduces a L1 cache (simple hashref). It will
be populated by accessing the L2 cache (Koha::Cache).
If a pref is retrieved 10x, the first one will get the value from the L2
cache, then the L1 cache will be check.
To do so we will need to clear the L1 cache every time a page is loaded.

Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com