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.

channeldb/db.go

@@ -59,6 +59,14 @@ var (
 			number:    3,
 			migration: migrateInvoiceTimeSeriesOutgoingPayments,
 		},
+		{
+			// The version of the database where every channel
+			// always has two entries in the edges bucket. If
+			// a policy is unknown, this will be represented
+			// by a special byte sequence.
+			number:    4,
+			migration: migrateEdgePolicies,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over