Bug 24642: Make Cache::Memcached::Fast::Safe required

Bug 24642: Make Cache::Memcached::Fast::Safe required

There is something wrong between the logic in Koha::Cache->new and our
dependencies.

We are having Cache::Memcached required, but
Cache::Memcached::Fast::Safe is optional.
However, in Koha::Cache->new we are initializing the Memcached cache
only if Cache::Memcached::Fast::Safe is installed.

We could fix the logic in Koha::Cache, but it seems better to make
Cache::Memcached::Fast::Safe mandatory (especially after bug 13193).

Note that after bug 13193, you are loosing the Memcached cache, as
Cache::Memcached::Fast::Safe will not get installed by default.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>