Bug 8089: Cache sysprefs with Koha::Cache::Object

Bug 8089: Cache sysprefs with Koha::Cache::Object

This patch replaces the %sysprefs hash variable with a
Koha::Cache::Object-tied hash that allows the user to cache sysprefs
in memcached, fastmmap, or memory.

To test:
1) Enable the cache as described on bug 9434.
2) Try viewing some pages in the OPAC and staff client. If the syspref
   caching isn't working, you'll be redirected to the web installer
   (in the staff client) or be shown the maintenance page (in the OPAC).
3) Disable the cache by unsetting the CACHING_SYSTEM environment
   variable.
4) Try viewing some pages in the OPAC and staff client. If the syspref
   caching isn't working, you'll be redirected to the web installer
   (in the staff client) or be shown the maintenance page (in the OPAC).

Note that this will not improve performance. It does, however, eliminate
the problem of sysprefs not getting updated in all threads in
multi-threaded environments.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>