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>