Add tx fee to the db and track for webhooks (#71)

src/scanner.cpp

@@ -394,7 +394,8 @@ namespace lws
               mask,
               {0, 0, 0, 0, 0, 0, 0}, // reserved bytes
               db::pack(ext, payment_id.first),
-              payment_id.second
+              payment_id.second,
+              tx.rct_signatures.txnFee
             }
           );