Instead of always performing the setup for the main domain, the setup
can now be performed for subdomains as well.

Instead of always performing the cleanup on the main domain, the task
now accepts a sdap_domain structure to perform the cleanup on. This
change will make the cleanup task reusable for subdomains.

The LDAP cleanup request was asynchronous for no good reason, probably a
leftover from the days of async sysdb. This patch makes it sychronous
again, removing a lot of uneeded code.

https://fedorahosted.org/sssd/ticket/1942

Identity providers other than LDAP need to customize the enumeration in
different ways while sharing the way the task is scheduled etc. The
easiest way to accomplish it is to leverage the recently introduced
ptask framework.

The LDAP enumeration was too closely tied to the LDAP identity provider.
Because some providers might need special handling such as refresh the
master domain record before proceeding with the enumeration itself, this
patch splits the request itself to a separate async request and lets the
ldap_id_enum.c module only configure this new request.

Also move the enum timestamp to sdap_domain to make the enum tracking
per sdap domain. The cleanup timestamp will be moved in another patch.

The constant was not used since Euegene came up with his reconnection
logic.

The enum code was quite old and predated the tevent_req style. In
particular, the enum code was checking tevent state direcly and not
using _recv functions or the helper macros we added later.
As a consequence, it was not easy to read. This patch adds the standard
_recv functions to read the status of the enum requests.

If USN attribute is not present, we call strdup on uninitialized
variable. This may cause segfault, or if we are lucky and
usn is NULL it will return ENOMEM.

Resolves:
https://fedorahosted.org/sssd/ticket/2052