CB subnet merge vs duplicate subnet prefixes

Current CfgSubnets4::merge method checks duplicate subnet IDs (erase the existing one before push back the new one) but not duplicate prefixes (subnet fields). I guess that the push back will throw, at least it fails as the prefix is an unique index in the multi index container.

It is a corner case because it is unlikely to get two subnets with exactly (ref #36 (closed)) the same prefix but different IDs. But it is still possible and the correct action when it happens needs to be defined.

#481 (closed) is related but not a duplicate.