In vanilla quic there isn't much to do when a major network change is detected
because there is a single active path. Quinn's process so far has simply been
to ping the path. In multipath this isn't as simple, because paths may be
affected differently by network changes. We should try to find a way to recover
connectivity faster/smarter.

As an issue per se, what we see is that even if winn is told of a network
change, paths will continue to be used as normal, creating a lot of packet
loss. The network change is always detected fromn "a higher level" (beyond
quinn-prot0, at least), so the caller could provide information on which paths
are still usable to reduce packet loss.