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.

lnrpc/lightning.proto

@@ -3330,6 +3330,9 @@ message RoutingPolicy {
 
     // Custom channel update tlv records.
     map<uint64, bytes> custom_records = 8;
+
+    int32 inbound_fee_base_msat = 9;
+    int32 inbound_fee_rate_milli_msat = 10;
 }
 
 /*