Local session cache and systemd PrivateTmp

With systemd, default tmp directory is not /tmp, but /tmp/systemd-private-xxxx-apache2-xxx/tmp

This is a problem for purgeLocalCache cron job which do not run with systemd, and can't purge the local cache, as it looks into /tmp to find sessions.

One solution is to disable PrivateTmp in apache2 systemd configuration, but this is out of LL::NG scope (not managed by our packages).

We could maybe find a better way to purge the local cache.