LDAP: new SDAP domain structure
    
    Previously an sdap_id_ctx was always tied to one domain with a single
    set of search bases. But with the introduction of Global Catalog
    lookups, primary domain and subdomains might have different search
    bases.
    
    This patch introduces a new structure sdap_domain that contains an sssd
    domain or subdomain and a set of search bases. With this patch, there is
    only one sdap_domain that describes the primary domain.