Get rid of node->wildcard_child field.

Remove the reference to wildcard child of a node. Instead, use a flag to mark nodes which do have a wildcard child. This will reduce the amount of searches in the zone to minimum - only when there actually is a wildcard that can cover the searched name.

While checking for wildcard child is done quite often - every time a name is not found in a zone - in most of these cases there is no wildcard child and thus the flag suffices to check it.