lnwire+channeldb: parse inbound fees

In this commit, the tlv extension of a channel update message is parsed.
If an inbound fee schedule is encountered, it is reported in the
graph rpc calls.

channeldb/models/channel_edge_policy.go

@@ -70,7 +70,7 @@ type ChannelEdgePolicy struct {
 	// properly validate the set of signatures that cover these new fields,
 	// and ensure we're able to make upgrades to the network in a forwards
 	// compatible manner.
-	ExtraOpaqueData []byte
+	ExtraOpaqueData lnwire.ExtraOpaqueData
 }
 
 // Signature is a channel announcement signature, which is needed for proper