cache_req: encapsulate output data into structure
    
    In enumeration calls we want to get objects from all domains, not
    only from the first matched domain. We move the cache search result
    into a structure that contains combination of domain and ldb_result.
    
    This is preparation for enumeration support inside cache_req.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>