Fixes https://fedorahosted.org/sssd/ticket/1766

Remove code that tries to get the 'right' sysdb, as it is always going
to get the same answer anyway since the recent patches to rework the
domains/sysdb relationship.

- Use a double-linked list for domains and subdomains.
- Never remove a subdomain, simply mark it as disabled if it becomes
  unused.
- Rework the way subdomains are refreshed.
  Now sysdb_update_subdomains() actually updates the current subdomains
  and marks as disabled the ones not found in the sysdb or add new ones
  found. It never removes them.
  Removal of missing domains from sysdb is deferred to the providers,
  which will perform it at refresh time, for the ipa provider that is
  done by ipa_subdomains_write_mappings() now.
  sysdb_update_subdomains() is then used to update the memory hierarchy
  of the subdomains.

- Removes sysdb_get_subdomains()
- Removes copy_subdomain()
- Add sysdb_subdomain_delete()

Use this function instead of explicitly calling domain->next

This function allows to get the next primary domain or to descend into the
subdomains and replaces also get_next_dom_or_subdom()

https://fedorahosted.org/sssd/ticket/1805

Also remove sysdb_delete_domuser()

Also remove sysdb_store_domuser()

Also remove unused sysdb_search_domgroup_by_gid()

Also remove unused sysdb_search_domuser_by_uid()