Bug 13193: Make Memcached usage fork safe

Bug 13193: Make Memcached usage fork safe

When a high enough number of forks try to access for example system
preferences with Koha::Cache using memcached as backend the results of
different cache requests get mixed up.

The problem is fixed by using Cache::Memcached::Fast::Safe that is a
fork safe version of Cache::Memcached::Fast.

Sponsored-by: The National Library of Finland
Signed-off-by: David Cook <dcook@prosentient.com.au>
Works as described, and solves an insidious difficult to debug
problem in Koha.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>