Use path-validation result from off-path path challenges

[flub]

During QNT the server performs off-path path challenges for nat traversal. If these produce a response however the result is ignored.

Instead the validation state should be stored somewhere so that when the client opens a new multipath path on that netwwork path, because nat traversal evidently succeeded, the server does not need to re-validate this path.

Note that the order is not always going to be in favour of this optimisation helping. It is entirely possible that the first nat-traversing packet is the path challenge from the client which opens the new path, while the server's off-path response was blocked by the firewall. In this case the server would start a new path challenge on the opened path, together with the PATH_RESPONSE, and this challenge would be more likely to validate the network path first.