Title: #390: NSS: Add option to tune the memcache size
Do we still want to have those changes merged at some point?
Yes, but there was condition that we need a way to get statistics about the cache usage,
so that users know when to make the cache bigger based on some information that we can
See the full comment at https://github.com/SSSD/sssd/pull/390#issuecomment-371428333