Fix race condition during zone loading.
    
    DNS zone has to be added to DNS view before dns_zone_load() is called.
    It is necessary to prevent dns_zone_load() from racing with dns_zone_setview().
    
    This race condition sometimes prevents zone from being signed.
    Now the unsigned zone is visible until signing process is complete. This
    mimics BIND behavior for in-line signed zones.
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/56
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>