Node discovery uses wrong detection of when new paths are needed

[flub]

DiscoveryTask::needs_discovery uses the NodeInfo::last_received API which is not information that NodeInfo can supply. It is not obvious if it could right now, this needs to be looked at to understand what the condition is really needed for needs_discovery and how to provide this from the PathState and whether (and how) it should be exposed from the more public NodeInfo.

[divagant-martian]

could you add some info on what apis it uses that are wrong and why? if we don't solve the issue soon, by the time we get back to this it will be very hard to remember/understand what's the problem