Bug 12041 - improve Koha::Cache

Bug 12041 - improve Koha::Cache

This makes Koha::Cache behave better by default. It will use memcached
if available to do shared caching, if that's not available it will fall
back to in-memory caching. It also allows for a singleton accessor to
allow a single cache to be shared within a process.

* Added tests to confirm UTF8-cleanness.
* Added minor fixups to stop warnings.

Test plan:
* The t/Cache.t file runs successfully with and without the
  MEMCACHED_SERVERS envvar set (and memcached running in the
  environment.)

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>