channeldb: store unknown policies in database

The commit ensures that for every channel, there will always
be two entries in the edges bucket. If the policy from one or
both ends of the channel is unknown, it is marked as such.

This allows efficient lookup of incoming edges. This is
required for backwards payment path finding.

routing/router.go

@@ -2136,6 +2136,10 @@ func (r *ChannelRouter) ForAllOutgoingChannels(cb func(*channeldb.ChannelEdgeInf
 	return r.selfNode.ForEachChannel(nil, func(_ *bolt.Tx, c *channeldb.ChannelEdgeInfo,
 		e, _ *channeldb.ChannelEdgePolicy) error {
 
+		if e == nil {
+			return fmt.Errorf("Channel from self node has no policy")
+		}
+
 		return cb(c, e)
 	})
 }