ipa-sam: cache gid to sid and uid to sid requests in idmap cache
    
    Add idmap_cache calls to ipa-sam to prevent huge numbers of LDAP calls to the
    directory service for gid/uid<->sid resolution.
    
    Additionally, this patch further reduces number of queries by:
     - fast fail on uidNumber=0 which doesn't exist in FreeIPA,
     - return fallback group correctly when looking up user primary group as is
       done during init,
     - checking for group objectclass in case insensitive way
    
    Patch by Jason Woods <devel@jasonwoods.me.uk>
    
    Reviewed-by: Alexander Bokovoy <abokovoy@redhat.com>
    
    https://fedorahosted.org/freeipa/ticket/4234
    and
    https://bugzilla.redhat.com/show_bug.cgi?id=1073829
    https://bugzilla.redhat.com/show_bug.cgi?id=1074314
    
    Reviewed-By: Sumit Bose <sbose@redhat.com>