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/devrpc/dev.swagger.json

@@ -217,6 +217,14 @@
             "format": "byte"
           },
           "description": "Custom channel update tlv records."
+        },
+        "inbound_fee_base_msat": {
+          "type": "integer",
+          "format": "int32"
+        },
+        "inbound_fee_rate_milli_msat": {
+          "type": "integer",
+          "format": "int32"
         }
       }
     },