[SSSD] Design Discussion: Periodic task API

[Pavel Březina]

I've been assigned ticket https://fedorahosted.org/sssd/ticket/1713:
[RFE] Add a task to the SSSD to periodically refresh cached entries

I have recently created a ticket (#1891) to unite API for managing 
periodic tasks. We already quite a periodic task (enumeration, sudo, 
dyndns, #1713) when each of them implements custom API.

None of these are generic enough to be used for #1713 so I will have to 
create a new one. I'm not suggesting to refactor the old code now, that 
will be done when #1891 is scheduled.

But I think it is a good idea to create the generic one now instead of a 
new feature-specific. It will be basically the same amount of work.

I wrote a design document:
https://fedorahosted.org/sssd/wiki/DesignDocs/PeriodicTasks