Bug Description: Import task should not be deleted too rapidely after import finishes
to be able to query the status.

Fix Description: A new attribute 'ttl' is order to tune the life time of the task.
The default value is increased to '86400'. Added a test to check that and added it
to ImportTask & ExportTask classes in lib389 so it will create ttl attribute by default.

Fixes: https://pagure.io/389-ds-base/issue/50177

Review by: mreynolds (Thanks!)