gossip store corruption with duplicate gossip_store_private_update

[rustyrussell]

Multiple 0.12 reports of gossip_store getting discarded on startup. Both cases I looked at contained duplicate private channel updates (which we detect and consider to invalidate the store: the first one should have been marked deleted!)

In both cases the timestamps were significantly different (maybe created on restart?) but otherwise non-signature identical (well, one had the disable flag set on the second one, not the first).

The workaround is simple (just complain and replace the old one), but I can't figure out how this is happening!