Though BOLT 7 says a channel may be pruned when one side becomes inactive
and fails to refresh their channel_update, in practice, the
channel_announcement can be difficult to recover if deleted entirely.
Here the channel_announcement is tagged as zombie such that gossip_store
consumers may safely ignore it, but it may be retained should the channel
come back online in the future. Node_announcements and channel_updates may
also be retained in such a fashion until the channel is ready to be
resurrected.
Changelog-Fixed: Pruned channels are more reliably restored.