It used to check if a net_id was ever assigned to that node to detect
when to send the path confirm to the remote peer.
This is wrong, because the same net_id is shared for all the remote
peers, but sent one by one.
Instead we now check if it's either not assigned or if the assigned
net_id is a cache ID, and in that case ensure that the remote peer has
been notified.
This can be further improved by unifying the cache interface, but for
now it's a fast fix to get path-only sync to work.
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