Stamp every registered connection and in-flight dial with a network
generation, bumped by MarkNetworkChange, which replaces the immediate
full sweep with one delayed by a configurable 500ms. The sweep then
cuts only registrations older than the last change: subsystems that
redialed on their own hold fresh-generation connections and survive,
so the callers no longer need cancellation logic around the sweep.
A connection inherits its dial's generation, because the socket was
bound to the network that was default when the dial started.
This fixes the sweep being cancelled by the engine's management-level
reconnect while the relay was still down, and lets the mobile
notifiers shrink to plain forwarders.
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