sysdb: store enumeration expiration time in autofs map
    
    We need to distinguish between 'object expired' when we need
    to obtain only the map entry and 'enumeration expired' when
    we need to enumerated the map entry.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/2607
    
    Reviewed-by: Tomáš Halman <thalman@redhat.com>