FO: Use tevent_req_defer_callback() when notifying callers
    
    If a fo_resolve_service callback would modify the server->common member
    in any way, for example by dereferencing the server and lowering the
    refcount to 0, which would free the common structure, then the next
    iteration of fo_resolve_service_done would access memory that was
    already gone.
    
    Please see
    https://tevent.samba.org/group__tevent__request.html#ga09373077d0b39e321a196a86bfebf280
    for more details.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit a92f68763a57b211a1bf6b80b6dd80c4a1aa2738)